Introductionntroduction book I

My principal aim in presenting these photographs has been to give pictures of some of the most interesting portions of the Milky Way in such form that they may be studied for a better understanding of its general structure. They are not intended as star charts. Such photographic charts have already been made by Wolf and Palisa and by Franklin–Adams. They are probably more useful for the identification of the individual stars. But these do not give us a true picture of the parts of the sky shown, for there are structures and forms that cannot well be depicted in ordinary charts, and it has seemed to me that some of these are of the utmost importance in the study of the universe at large. These photographs may, therefore, be considered as supplementary to the regular charts in that they show the details of the clouds, nebulosities, etc. In this form, however, it is always difficult to identify the individual small stars. To overcome this difficulty charts have been prepared corresponding to each photograph and giving on the same scale a set of co–ordinates, and all the principal stars and objects of especial interest. The most useful reference stars are numbered, as are the dark objects. These charts and the tables, which give fuller data about the reference stars, will be found in Part II. It is recommended that in studying any photograph the reader should open Part II to the corresponding chart, and then he will have before him the photograph or plate, the author's text descriptive of it, the chart, with its co–ordinates, including most of the stars of the Bonner Durchmusterung, and the table supplementary to the chart.

The Milky Way has always been of the deepest interest to me. My attention was first especially attracted to its peculiar features during the period of my early comet–seeking. Indeed, there is no work in observational astronomy that gives one so great an insight into the actual heavens as that of comet–seeking. The searcher after comets sees more of the beauties of the heavens than any other observer. His telescope, though small, usually has a comparatively wide field of view, and is amply powerful to show him most of the interesting parts of the sky. To him the Milky Way reveals all its wonderful structure, which is so magnificent in photographs made with the portrait lens. The observer with the more powerful telescopes, and necessarily more restricted field of view, has many things to compensate him for his small field, but he loses essentially all the wonders of the Milky Way. To me the views of the galaxy were the most fascinating part of comet–seeking, and more than paid me for the many nights of unsuccessful work. It was these views of the great structures in the Sagittarius region of the Milky Way that inspired me with the desire to photograph these extraordinary features, and one of the greatest pleasures of my life was when this was successfully done at the Lick Observatory in the summer of 1889.

Description of the Bruce Photographic Telescope I

My experience at the Lick Observatory with the Willard portrait lens impressed me with the importance of that form of instrument for the picturing of large regions of the heavens.

That lens, which was purchased at second hand from a photographer in San Francisco, was made for, and originally used in, taking portraits — from which fact its name has come. These large short–focus lenses were necessary in the days of wet–plate photography to gather a great quantity of light and to give a brilliant image to lessen as much as possible the time of sitting. But when the rapid dry plates came into use these lenses were no longer needed, and much smaller, more convenient, and less expensive lenses took their place. The great light–gathering power for which they were so valuable in the wet–plate days makes them specially suitable for the photography of the fainter celestial bodies. They were made on the Petzval systems and consisted of two sets of lenses, from which fact they are also called "doublets." In this paper I shall refer to them namely as "portrait lenses," as that name appeals more directly to me.

The main advantage of the portrait lens lies in its grasp of wide areas of the sky and its rapidity of action – this last result being due to its relatively short focus. The wide field makes it especially suitable for the delineation of the large structural details of the Milky Way; for the discovery of the great nebulous regions of the sky; for the investigation of meteors and the determination of their distances; and especially for the faithful portrayal of the rapid changes that take place in the forms and structures of comets' tails.

The portrait combination is not intended in any way to compete with the astrographic telescopes, or with any of the larger photographic refractors or reflectors. It must be considered as supplemental to these, because their limited field confines them to small areas of the sky. There is a great and valuable work for these larger telescopes, however, in the accurate registration of the places of the stars, for parallax, and, in the reflector, for depicting the features of the well–known nebulae, etc.

There is, I think, however, a question as to the most advantageous size for a portrait lens, and I have believed that the best results can be obtained with an instrument of moderate size; or, in other words, I believe that a portrait lens can be made too large to give the very best results, just as it can be too small. It is also true that both large and small portrait lenses are individually valuable. There is a kind of supplementary relationship between them. The small one will do work that the large one cannot do' and the reverse of this is equally true; for though the small one is quicker for a surface – such, for instance, as the cloud forms of the Milky Way present to it – the larger one, mainly on account of its greater scale, will show details that are beyond the reach of the smaller one. Another important fact is that as the size of the lens increases, the width of the field rapidly diminishes, and width of field is one of the essential features of the value of the portrait lens.

There would, therefore, seem to be a happy mean, when the available funds limit the observer to one lens only.

As a matter of experience, it has seemed to me that a lens of the portrait combination about 10 inches in diameter would best serve the purpose of the investigations that have just been outlined.

For several years I had tried to interest someone in the purchase of such a lens, but without success. Finally, I brought the matter before Miss Catherine W. Bruce, who had done so much already for the advancement of astronomy. In the summer of 1897 Miss Bruce placed in my hands, as a gift to the University of Chicago , the sum of $7,000 for the purchase of such an instrument and for the erection of a small observatory to contain it.

The instrument consists of a 5–inch guiding telescope and two photographic doublets of 10 and 6 ¼ inches aperture, rigidly bound together on the same mounting. An unusual delay was produced by my anxiety to get the best possible lens for the purpose.

The long exposures demanded in the work of an instrument of this kind require an unusual form of mounting to give an uninterrupted exposure. The mounting of the Willard lens was an ordinary equatorial and was not made specially for it. It did not permit an exposure to be carried through the meridian, except in southern declinations. This was a great drawback since in a long exposure it was necessary to give all the time on one side instead of dividing it up to the best advantage on each side o the meridian.

There were two forms of mounting in use that would permit a continuous exposure. These were (1) the English form of equatorial mounting, which is a long polar axis, supported at each end with the tube swung near the middle; (2) the Potsdam astrographic equatorial, in which the polar axis projects far enough to allow the telescope to swing freely under the pier. Neither of these mountings has appeared to me to be entirely the best form for the purpose.

With the short length of this instrument it seemed that if the pier itself were bent to form the polar axis, the telescope could be made to swing freely under the pier in all positions. With this idea in view, I went to Cleveland to confer with Messers. Warner and Swasey on the matter. Mr. Swasey at once took the deepest interest in the proposed telescope, and eventually evolved the scheme that was ultimately adopted in the mounting. The result was entirely satisfactory, and the mounting is, I believe, the best for the purpose that has yet been made.

The next question was the lens, and here is where the delay occurred. It was my wish to get the widest field possible and shortest relative focus consistent with such a field. This proved to be a problem of the most extreme difficulty. Dr. Brashear, who was appealed to for the optical part, entered heartily into the subject. So earnest was he in his endeavors to fulfill the required conditions that he made at least four trial lenses at 4 inches diameter and upward. But my ideal was evidently too high and one not attainable with optical skill.

In the interests of the matter I made a visit to Europe to see if better results could be had there, but, in the end, it proved that Brashear's lenses more nearly fulfilled the requirements than any that I saw elsewhere.

In the meantime, Mr. Brashear, with characteristic faith in his skill, ordered the glass and made a 10–inch doublet on his own responsibility. This lens gave exquisite definition over a field some 7° in width and could by averaging be made to cover at least 9° of fairly good definition. Though this did not come up to the width of field originally proposed, it was finally accepted, as it seemed the best that could be obtained.

The glass disks were made by Mantois, of Paris , and delivered to Brashear in May of 1899, and the lenses were completed in September, 1900.

The following information about the 10–inch lens was supplied me by Dr. Brashear:

The general construction is that which was first found by Petzval several years ago, and has proven itself quite the best where great angular aperture and sharp definition is imperative. The curves have been somewhat modified from our experience in the construction of other lenses – particularly of those made for Dr. Max Wolf, of Heidelberg , Germany . It departs, however, from the ordinary practice of opticians in being corrected for short wavelengths of light. This would be quite objectless in a camera which is to be used for portraits, but it is not without moment in astronomical photography.

The materials employed were specially chosen for their transparency – the flint being very light and the crown very white. The focal lengths of the front and rear combinations are in a ratio of about 7 to 12, while the focal length of the system is very nearly five times the aperture. The focal length you may find very slightly modified; indeed, it is our custom to balance the inevitable zonal differences of magnification to all constructors of astronomical objectives.

The focus of the 10–inch, determined form the photographs, is 50.3 inches (127.8 cm), and the scale is therefore 1 inch = 1°14 or 1° = 0.88 inch. The ratio, a/f = 1/5.03, I believe to be the best for the purpose.

The accumulation of interest had by this time permitted the purchase of a 6 ½–inch Voigtlander lens of 30.9 inches (78.5 cm) focus, which had been in commercial use.

As indicated, the telescope is really triple in character, there being three tubes bound rigidly together on the same mounting – the 5–inch visual telescope for guiding, and the 10–inch and 6 1/4 –inch photographic doublets. For each of the photographic lenses there is an inner tube, with focusing scale, which can be racked back and forth for the adjustment of focus. There is considerable change of focus in the 10–inch lens between winter and summer. The change in the focus of the 6–inch is small, however, and requires very little correction.

The plate–holder for the ten–inch carries a plate 12 inches square, while the one for the 6 ¼ –inch carries a plate 8×10 inches.

In the matter of a guiding telescope the limited means would not permit of anything larger than 5 inches, which is sufficiently powerful for ordinary purposes, though for the photography of comets a larger one would have been desirable. The guiding telescope I used with the Willard lens at Mount Hamilton was only 1 ¾ inches in diameter. Of course the question of a double–slide plate holder was considered; but in a small telescope like this the tubes are so rigidly bound together that such a device is not necessary to insure faithful guiding. Furthermore, for work of this kind the double–slide plate–holder would be seriously objectionable.

A high–power eyepiece is used on the 5–inch for guiding in conjunction with a right–angled prism. This is more convenient than direct vision, especially when photographing at high altitudes. The eyepiece has an adjustable motion to the extent of 2° in any direction, thus insuring the finding of a suitable guiding star. This is also valuable in photographing a comet, as it permits the displacement of the comet's head to one side of the center of the plate, thus securing a better representation of the tail.

Two spider–line cross–wires in the eyepiece are used for guiding. They are illuminated by a small electric lamp by the aid of two small reflecting surfaces which throw the light perpendicularly on the wires. The intensity of the illumination is readily regulated. By this means almost the smallest star visible in the 5–inch can be used for guiding purposes.

The illustration will give a better idea of the Bruce telescope than any mere words can do. Indeed, there are very few things about it that need explanation. One feature, however, will not be clear without a description, viz., the method of adjustment for latitude in case the telescope were removed to a different latitude. It was intended that the instrument should be portable when occasion arrived, for the purpose of observing eclipses, etc., and for possible transportation to the southern hemisphere.

The pier really consists of two parts. Just above the clock room is separates into two pieces which are bolted together on the inside of the pier, and hence no break appears in the continuity of the pier.

For change of latitude, it is only necessary to insert a wedge–shaped section between these two parts of such an angle that it will produce the required change of latitude. This ordinarily would necessitate only a slight change in the length of the driving–rod which is adjustable. No other means of adjustment seemed feasible.

As it was possible that the instrument might some time go to the southern hemisphere, Messers. Warner and Swasey were asked to insert some sort of gearing that would readily permit of a reversal of the motion of the clock. The device they introduced is extremely simple and efficient. In a couple of minutes' time the motion can be changed from west to east. At the point where the driving–rod joins on to the worm–screw for driving the worm–wheel carrying the telescope, the small gear–wheel which makes the connection can be reversed and placed on the other side of the gear–wheel at the end of the driving–rod; this will reverse the direction of the motion of the worm–wheel and hence of the telescope.

The telescope is supplied with fine and coarse right–ascension and declination circles; the fine circles are divided on silver and are read by verniers.

The slow motions for guising are brought down conveniently to the plate–end of the instrument.

The pier is very heavy, weighing some 1,200 or 1,300 lbs. (550–600 kilos). This great weight is necessary to support the overhanging mass of the telescopes and the top of the pier.

The driving–clock is of Warner and Swasey's regular conical pendulum pattern, which by all means seems to be the best form of driving–clock. It is a beautiful piece of mechanism and performs satisfactorily, though we intend to introduce an electric control for work with it hereafter.

The instrument was finally finished and placed in position in its observatory in April 1904.

The photograph shows the compact and rigid form in which the tubes are mounted, and it will at once be seen how the combination can swing freely under the overhanging pier.

As will be noted, the design is a new one, and although Messers. Warner and Swasey have made at least one mounting of this kind (for the Tokyo Observatory) before the Bruce telescope was commenced; it was made from their design for the present instrument, so that the Bruce is the original of this particular form of mounting.

As I have said, small portrait lenses have their special advantages as well as the larger ones. Where it is possible, it is desirable that two or more lenses should be used on the same mounting, a very important point being that they mutually verify each other. Duplicate lenses would not seem to be either the most economical or the best arrangement. In that case they would serve only as a verification and could have no other value, unless indeed one of the plates should meet with an accident or be defective – circumstances that would not be of sufficiently frequent occurrence to justify the extra outlay. The best plan would seem to be to have one of the instruments decidedly different from the other so that an independent series of pictures of the same region could be secured on a very different scale. Photographs with these, at the same time that they mutually verified each other, would have values peculiar to themselves.

The 10–inch and the 6 ¼–inch, therefore, mounted together, give a very desirable variety in respect to scale, at the same time that the 6–inch is sufficiently powerful to be an almost perfect verification of anything the 10–inch may show.

One minor source of trouble with both these lenses, but worse in the case of the 10–inch, is that the commercial plates that are used are never flat. In one sense this is a distinct advantage as the emulsion is placed on the concave side of the plates; this helps to flatten the field. But the curvature is not always the same, for some plates are curved more than others. This is equivalent to a frequent change of focus with the larger lens. Once in a very long while the emulsion is put on the convex side of the plate. This puts the sensitive surface too much inside the focus and the result is a spoiled picture.

The Bruce Observatory is a wooden building of size, 15×33 feet, with the greater length lying east and west. The dome, which is central, is 15 feet in diameter and revolves on 8–inch roller–bearing iron wheels.

The large field of the Bruce telescope made a wide opening in the dome a necessity. It was therefore made 4 feet wide, which seems ample for all purposes. The telescope rests on a brick pier, and the observing room is reached by a small stairway against the inner south wall of the building.

The altitude of the telescope above sea–level is about1,040 feet (317 meters). Its latitude is 42°34'.

The Work at Mount Wilson

Through the interest and courtesy of Professor George E. Hale and the generosity of Mr. John D. Hooker, of Las Angeles, I spent the spring and summer of 1905 in photographic work at the Solar Observatory of the Carnegie Institution on Mount Wilson , California . Mr. Hooker's generous grant made it possible to transport the Bruce telescope to Mount Wilson , where it was installed from February until September, 1905, in a temporary wooden structure, from which the roof could be slid off, giving an unbroken view of the sky. The altitude of the station was about 5,900 feet (1,800 meters), above the sea, and its latitude 34°13'.

The main object of this expedition to Mount Wilson was to secure the best possible photographs of the Milky Way as far south as the latitude would permit. But little time was available for independent investigations into other parts of the sky, though the conditions for such work were often superb. During this period 154 plates were obtained with the 10–inch Brashbear doublet, and 151 with the 6 ¼–inch Voigtlander doublet, the exposures being simultaneous, almost without exception. The original negatives of 40 of the 50 photographs in this volume were made during the time at Mount Wilson .

During many of the exposures at Mount Wilson two additional cameras were used, being attached to the mounting of the instrument, as shown in the picture. These were a Clark lens of 3.4 inches aperture and 20 inches focus and a so–called "lantern" lens of aperture 1.6 inches and focal length of 6.3 inches. With the Clark lens about 110 negatives were obtained and about 90 with the stereopticon lens.

General Remarks on the Milky Way

The development of astronomical photography, especially where portrait lenses are used, has brought to our knowledge the existence of large areas of faint diffused nebulous matter in different parts of the sky. Some of these have been shown by the stereoscope to be gaseous, while it leaves others either in doubt or distinctly not gaseous. As one is not called upon to decide as to the gaseous nature of this matter, it will be strictly correct to speak of it as "nebulosity." This term seems to have come into use or to have been adopted as more satisfactory and explanatory than the word "nebula," which is more readily applicable to the older known forms of the nebulae as seen with the visual telescope. It seems now to belong distinctly to those large, diffused areas of matter mostly shown on small–scale photographs within the last thirty years, such as those revealed in Taurus near the Pleiades and south of the Hyades and in Ophiuchus and the Scorpion, and in other parts of the sky. Though those are not strictly confined to the Milky Way they are generally found in connection with it, some of the finest being in the Milky Way itself. There seems to be some evidence of such masses being apparently connected with some of the brighter regions of the Milky Way, a large bed of it being found in ? ... near one of the smaller bright star clouds in Sagittarius and in the region of the star Gamma Cygni, where it appears in the form of nebulous tufts and masses over a large area, and in the region of the North American nebula.

While I was at Mount Wilson in 1905 I made a few exposures at various points in search for diffused nebulosities. The extraordinary nebulosities in Scorpio and Ophiuchus which I found by photography in 1894 – those of Rho Ophiuchi, Nu Scorpii, etc. – suggested the immediate region of the upper part of the Scorpion as a suitable hunting–ground. Trial plates were exposed on Rho Scorpii, Pi Scorpii, and elsewhere. The photographs of the region of Pi showed a very remarkable, large, straggling nebula extending from Pi to Delta Scorpii, with branches involving several other naked–eye stars near.

With the exception of the great curved nebula in Orion and some of the exterior nebulosities of the Pleiades, this nebula is quite exceptional in its content, and in the peculiarities of its various branches. A simple description of it would be inadequate to give a fair conception of these features. It is difficult to reproduce properly the photograph because of the faintness of some of the extensions of the nebula. Enough can be shown, however, to give some idea of its general structure (Plate 11).

From a long familiarity with the transparency of comets, we perhaps came too soon to the conclusion that the nebulae also are transparent. Unfortunately, it is not possible to either prove or disprove the transparency of the nebulae in the same manner as we do that of the comets, for the nebulae do not conveniently move about over the sky as the comets do. Though we cannot test this question by moving the nebulae over different parts of the sky, we can safely prove it by considerations almost as convincing. These nebulous masses often occur in regions where the sky is uniformly covered with stars, as in the case of the nebula about Nu Scorpii and the region of Rho Ophiuchi. In these cases there is a noticeable lack of stars within the confines of the nebulosity and in some cases a total disappearance of them as if their light was cut out by the intervening nebulosity. An inspection of these photographs, therefore, seems to show that the same nebula may be partly or totally transparent. Also, the less luminous parts seem to be the more opaque. Frequently there is a curious apparent mixture of stars and nebulosity – a free mixture, one might say – where though seemingly mixed together there is no apparent condensation of the nebulosity about any of the stars. This apparent association without visible connection happens too frequently to be due to chance.

Some of these, such as the nebulosities exterior to the Pleiades, and elsewhere, are of such irregular brightness as to compel attention. But there are other regions in which a film of this faint nebulosity uniformly covers the sky for considerable distances. From the wide and uniform distribution of this nebulosity it is not always possible to prove its existence because it covers the entire plate uniformly and cannot be distinguished from the sky–fogging always present on long exposures. But there are certain cases where a dark body projected against it is unmistakably revealed. A very striking case of this kind occurs in Sagittarius in the region of the small, bright star cloud in ? = 18 h 8 m , d = –18°. In this star cloud (shown on Plate 31) are two black spots, the western of which is the more conspicuous and definite. I have already shown that this spot is a real dark object seen by contrast with the brighter region against which it is projected. On the original negative the eye at once picks this object out as being the darkest part of the entire plate. Such effects are sometimes produced by contrast and may not be real. I have cut holes the size of this spot in a black paper mask with openings of the same size. With one of the openings over the spot, excluding the stellar background, it is readily seen that this spot, by comparison with other parts of the sky equally free of stars, is very much darker than any other part of the plate. Furthermore, the outline of the eastern edge of the spot is sharply defined, not against the stars but against a thin film of more luminous material. There is scarcely a star close to this outline. This thin, lighter film against which we see the spot permeates the entire star cloud and the rest of the plate. It is this nebulosity that makes the star cloud so conspicuous and not the abundance of stars.

In regard to a region of diffused nebulosity near Omicron Persei I quote from an article of mine in the Astrophysical Journal (41, 253–258, 1915):

Attention has been called frequently in this journal and elsewhere to regions of this kind which are of special interest – where apparently an intimate connection exists between the vacancies and the large masses of nebulosity. It has been shown in these papers that there is evidence of the existence of some kind of dark or partly luminous matter between us and the fainter stars which, by obscuring the stars, produces the apparent vacancies, and that the diffused nebulosities, referred to above, are the visible evidence of this matter. Regions of this kind were found in Scorpio, in Ophichus, and in Taurus. Inn Publications of the Lick Observatory, 11, Plate 16 , I called attention to the lower right corner of that photograph. Some of the nebulosity is shown faintly, and it was suggested that a long exposure at that point would perhaps show more of this matter, its presence being indicated by the otherwise unexplained absence of the small stars.

On November 21, 1914 , I gave an exposure of 6 hours and 41 minutes on this region with the Bruce 10–inch and 6–inch telescopes. A large, feebly luminous nebulosity with considerable detail in is shown on these plates. The more obscure parts of this nebulae are excessively faint, but the brighter details are well shown. The nebulae fits into the vacancy referred to and seems (by obscuring their light) to account for the absence of the small stars. It will be noticed, as in other cases to which I have called attention, that in the brighter part of the nebula west of Omicron Persei the background of small stars is continuous. It is only where the nebulosity is very feeble that the stars seem to be more of less missing.

The photograph referred to above is reproduced as Plate 3 of this Atlas.

This region of Omicron Persei is intimately connected with the more remarkable one shown on Plate 5, which lies south and east of the present object. The dark lanes in this region in Taurus seem to be due mainly to an abrupt absence of stars. They are so distinct and definite that they look artificial, as if they had been made with a stencil. They occur in a luminous region against which they appear in strong contrast, though broken in parts of their length. The strange thing is that the small stars, which are so thickly strewn over the sky here, seem, with few exceptions, to have disappeared, as if the "lanes" had hidden them. Though they are free from stars they apparently are not free from the faint nebulosity.

The faintly luminous film that covers all of the southern half of the plate seems to be beyond the general stratum of stars, for all the stars appear to shine on or in it. The lanes appear to be due in part to the absence of stars. At the same time they seem also to be in the substratum of nebulosity. In places they become blacker than the background on which they appear. This is specially noticeable in the great, partly dark nebula itself, for it is very much darker than the sky against which the stars are seen. In fact, the dark lanes seem to do two things – they blot out the stars, and at certain places they blot out the feeble nebulous background on which the stars shine. Here, as in many other places, one gets the impression that the stratum of stars is not very deep or thick (see ibid., 25, 218–225, 1907.)

Some of the dark markings of the catalogue, which follows, may be only vacancies among the stars, but I have tried to avoid such as much as seems possible. In many cases, however, there seems to be no other interpretation of the appearance than that of an obscuring body. In some cases the dark body itself can be distinctly seen on the photograph, such as Nos. 33, 72, 133, and others, so that there need be no hesitation in accepting the fact that such bodies exist.

One reason why I often think that a small vacant region is an obscuring body, even though a long exposure does not show it, is that in several cases where a long exposure does show such an object, a short exposure simply shows a vacancy with no real suggestion of an obscuring body. I therefore, think that in many cases it only requires a much longer exposure to show the real object.

There are two classes of these dark spots. Some are merely gray and devoid of stars. Others are extremely black and still others are a combination of there, a gray vacancy with a very black spot in it.

The smallness of some of these objects and their definite form led me to examine a few of them with the 40–inch refractor under suitable conditions. In each case it was shown that a real object of an obscuring nature was present. The results of these visual observations will be found in ibid., 38, 496–501, 1913, and 49, 1–23, 1919.

In my list of dark objects there are several that are seen to be identical with some of those in the list of starless fields given in Webb's Celestial Objects, Volume 2, Appendix I, taken from Sir John Herschel's observations made at the Cape.

TABLE (10)

In this volume attention is called to various peculiarities found on the photographs, such as thin, dark lanes of uniform width among the stars, curves and straight lines of small stars, often of equal magnitude. I am aware of the fact that these singular features are believed by many to be fortuitous and that strikingly similar figures can be reproduced by artificial means. While it is possible that they have no meaning in reality, there is a probability that many of them are real and are due to some law that forces such alignments upon the stars. Attention has been called to the most striking of these features so that should it ever be desirable to investigate them, there will be ample material to work on. It is probable that some are due purely to chance, and that others are real and are due to some law that will reveal itself in the course of time.

The Milky Way as Seen With the Naked Eye

At different times I have tried to visualize the Milky Way and to describe its appearance with the eye alone. It is extremely difficult to do this satisfactorily, mainly because of the indefinite limits to certain portions of it.

Even when a small boy I was struck with the difference in the brightness of the sky to the east and to the west of the Milky Way. The difference is very striking when we compare the sky on opposite sides of the Milky Way near and north of Orion. While the sky is rich and black to the west, at a similar distance to the east it is luminous, so that one cannot locate the eastern limits which seem to extend indefinitely. Not only does this hold in the winter regions of the Milky Way, but it is also noticeable on summer nights, or in the opposite parts of the heavens, where the effect is equally striking. It seems to be a fact that the western side of the Milky Way is almost indefinitely diffused while the opposite side is less diffused, or, in other words, the Milky Way extends farther northward from the plane of the galaxy in the region of Orion and farther southward in the region of Sagittarius. This feature must have been noticed by others, but I have seen no reference to it.

The "coal–sack" north of Alpha Cygni appears blacker than any part of the visible heavens except perhaps farther to the west – 30° or so – where the sky is darker. Under exceptional circumstances, when the sky is very transparent, the great dark space that runs to the west under Theta Ophiuchi is a noticeable object to the naked eye. Even in a poor sky, but free from moonlight, its presence is evident.

The beautiful star cloud in which Messier 11 is placed is a striking feature with the naked eye, though its true form is scarcely made out. The small star cloud in ? = 18 h 10 m , d = –18°40', Messier 8, and the Trifid nebula are also noticeable to the eye.

The region in Sagittarius, however, contains the finest and brightest portions of the Milky Way that are seen from the northern hemisphere. The stars pile up in great cumulous masses like summer clouds. The extreme brilliancy of these great star clouds in Sagittarius is never better shown than on a heavily clouded moonless night when holes or small breaks occur in (terrestrial) clouds. At such times when these openings pass over Sagittarius the glimpses seen through the breaks appear very bright as if an illumination far greater than the Milky Way was shining through the openings. I have often seen it thus and wondered as its brightness. The reverse of this sometimes occurs when the sky is clear and a few minute clouds happen to be seen against the great star clouds. Then we have a vivid representation of the black spots found in photographs of these bright clouds. I quote from my article in Astrophysical Journal for January, 1916 (43, 1–8):

All that is needed to make these dark bodies visible is a luminous region behind them. This is supplied in one way by the rich stellar regions of the Milky Way. An excellent example of how such a thing may be possible is shown by a phenomenon that presented itself to me one beautiful, transparent, moonless night in the summer of 1913, while I was photographing the southern Milky Way with the Bruce telescope. I was struck with the presence of a group of tiny cumulous clouds scattered over the rich star clouds of Sagittarius. They were remarkable for their smallness and definite outlines – some not being larger than the moon. Against the bright background they appeared as conspicuous and black as drops of ink. They were in every way like the black spots shown on photographs of the Milky Way, some of which I was at that moment photographing. The phenomenon was impressive and full of suggestions. One could not resist the impression that many of the black spots in the Milky Way are due to a cause similar to that of the small, black clouds mentioned above – that is, to more or less opaque masses between us and the Milky Way. I have never before seen this peculiarity so strongly marked from clouds at night, because the clouds have always been too large to produce the effect.

On examining the heavens with the naked eye, strange as it may seem, one does not notice any special increase of individual stars in approaching the Milky Way, nor are the stars brighter in general. Of course many of the brightest stars, such as Sirius and Canopus and those in Orion and Cassiopeia, are in the Milky Way, but this is doubtless purely accidental. If, however, the telescope – even a small one – is turned to the sky it is at once seen that as we approach the Milky Way there are many more stars, but it is not until a very powerful telescope or the photographic plate is used that we notice any large increase of stars in the Milky Way or its vicinity.

The Milky Way consists almost wholly of apparently small stars. If all the stars down to the twelfth magnitude, say, were removed from the sky the Milky Way would not be sensibly altered in appearance but it would shine on a dark, unbroken sky. There would not be an individual star visible anywhere on the blackness of space, and the Milky Way would appear as it does now, shorn of a few bright naked–eye stars. Its appearance is due entirely to the light from stars wholly beyond the naked eye and, we can say with assurance, much beyond the reach of ordinary field glasses.

To the naked eye the Milky Way presents a different appearance from that shown by the photographic plate. It does not seem possible to reconcile them entirely. The main difference is due to the penetrating power of the photographic plate and the smallness of the field of the lens used to make the photograph. Pictures taken with a small lens, like that used for Plate 51, covering larger areas of the sky, more nearly approach the appearance presented to the naked eye.

Preparation of the Reproductions

Immediately after the grant for the publication of this Atlas was assured by the Carnegie Institution of Washington, investigations were begun as to the best available methods of reproduction. With the cordial co–operation of some firms, experiments were made with the photogravure and other processes. It was feared, however that these methods, though capable of reproducing the sky, might depart from the fidelity necessary in work like this. It was also questioned whether the quality of the reproduction could be maintained throughout the whole edition. After long consideration it was decided that the most faithful reproduction could be secured by using photographic reproduction from second negatives especially prepared for the purpose by copying the original negatives.

There is much to be said in favor of this process as being the best one for reproducing correctly the original photographs. But it also has its limitations, as all things have where the judgment of the human mind is a large factor. The printing is done with a photographic printing machine, where the exact exposure can be automatically controlled, so that each print is given exactly the same exposure time. That part is under perfect control. But this is not all. The development of the prints must depend on the judgment and skill of the operator. If the development is cut short, the proper strength and softness is not secured, while a little too much development will loose the faint nebulosities and details which are the most valuable parts of the photograph. The development apparently cannot be made automatic in a case of this kind to give reliable results. We must depend on the judgment of the operator.

I have personally examined each one of the 35,700 prints and have rejected all that were not up to a certain standard, or which had defects in them. But there were many cases where a rejection was unfair. This was when the print was slightly too dark or too light, but the difference was not large. Such a print must be passed, though the desire was great to throw it out. In other words, it seems impossible for the manipulator to attain to perfection in this work.

Much difficulty was at first experienced in getting perfect contact all over the plate in the printing, but this was finally overcome by the printer.

The difficult task of making these photographic prints was entrusted to Messrs. Copelin, commercial photographers of long experience, of Chicago . They devoted to the work great skill and patience during the years 1915, 1916, and 1917, and the author made frequent visits to the city to inspect the sets of prints as they were completed. Those who are best acquainted with celestial photography will appreciate most fully how successfully Messers. Copelin have accomplished their undertaking. The author would express his most sincere thanks to them for their unfailing courtesy and constant desire to meet his exacting requirements that the qualities of the original negatives should be reproduced as perfectly as possible.

There is always a question as to the permanency of a photographic print. Absolute permanency is perhaps not attainable with any process of reproduction. I have been assured that every precaution was taken to insure permanency, both in the fixing and the washing of the prints, for these are two great sources of uneasiness on the score of permanency. The actual permanency of the paper itself is another source of anxiety. Tests were made as to the effect of light upon them by exposing them to open sunlight for several months. It was found that the paper into the emulsion of which it had been introduced a certain dye for color effect was subject to fading. That paper was rejected, thought it took away some of the effectiveness of the prints. Unless there is some change from a chemical standpoint in the paper itself, and this can be tested only by lapse of time, I think the prints will be reasonably permanent.

It may be interesting to know how these prints are mounted and burnished. After the final washing the prints are placed, while wet, face down on a ferrotype plate which has a brilliant surface. They are pressed firmly on to this by rubbing out the water and air bubbles with a rubber "squeegee." A piece of cheesecloth, after being covered with starch paste, is pressed and rubbed tight on to the back of the print, a strip of paper having been introduced to serve as a hinge for binding the print into the volume. The plate is then put to dry. When dry, the print thus mounted comes off freely and is beautifully burnished and ready, after trimming, to be bound in the volume. There is nothing new in the process, however.

In every case, for the printing of these plates a second negative was used. The original negatives do not give sufficient contrast or strength for printing purposes. It has been my custom, from the early days at Mount Hamilton , to use second negatives. To secure these the best possible positive is made from the original negative on, say, a Seed "Process" plate which will give a fairly strong but not harsh picture, showing the faintest details. IN general, this is strong enough to give a good printing negative with a Seed "23" plate, which will retain the softness and strength necessary for printing. It is a mistake, in general, to use for this second negative a slow plate, such as the Process plate, which only introduces harshness. It is sometimes necessary to use such a plate when the picture is not strong enough. Indeed, a third negative is occasionally, though seldom, required. In this case a second negative is prepared on a Seed 23 plate. From this a positive on a Seed 23 is made, keeping the softness and details as much as possible. This second positive will give a third negative of the required softness and density. It is almost never necessary to go beyond this stage. A good plan in making the first positive is to overtime the exposure slightly. Then develop up black – or overdevelop. The positive, after being fixed, should be slowly reduced with a weak solution of red prussiate of potash and hypo. This reducer has a tendency to produce contrast. A strong solution is apt to reduce unequally and to spoil the positive if the reduction is prolonged.

A uniform scale of enlargement has not been employed because the author felt that different areas required a different magnification to bring out their most interesting features to the best advantage. Therefore, the scale for each plate is given at the head of its description page.

The positions of the centers as written in the upper right corners of the prints are only approximate, and do not always agree exactly with the positions given elsewhere in type. The epoch for all positions is 1875.0, chosen because of the convenience of using the positions in the catalogues of the Astronomische Gesellschaft. For all the photographs, north is at the top. The times of the mid–exposures are given to the thousandth of a day in Greenwich Mean Time (reckoned from Greenwich Mean Noon).

The portions of the Milky Way which are included in the Atlas may be most easily seen by examining the diagram which shows their distribution according to galactic longitude and latitude. The centers of the plates are represented by the dots, with the corresponding numbers of the plates. The diameter of the field varies according to the enlargement, and ranges from a minimum of 3 1/2° to a maximum of 10 1/2°. Galactic longitudes between 230° and 310° were of course too far south to be reached from either the Yerkes Observatory or Mount Wilson . The concentration of plates in the regions of especial interest in Ophiuchus, Sagittarius, and Scorpio is at once apparent on the diagram. It was the author's deep regret that the necessary limit to the number of plates which could be reproduced in a volume like this did not permit the inclusion of a greater extent of the galaxy.

Here, as elsewhere in this book, the galactic positions assume the position of the north galactic pole to be at ? = 12 h 40 m , d = +28°0' for 1900.00.

1 Extracted from Professor Barnard's article in the Astrophysical Journal, 21 , 35–48, 1905.


Abbreviations Used

For the benefit of those not familiar with catalogues of stars and nebulae, it may be well to give some of the abbreviations used in designing such works.

B.D. denotes the Bonn Durchmusterung of the northern heavens which is comprised in Volumes III–V of the Astronomische Beobachtungen auf der Sternwarte der Universitat zu Bonn, by Dr. F.W.A. Argelander, 1859, 1861, and 1862, with its extension (Vol. VIII) to the zones –2° to –23°, by Dr. E. Schonfeld, 1886. The letters B.D. are followed by the zone in declination, with the number of the star in the zone.

The letters C.D. designate the Cordobas Durchmusterung of the southern sky, forming Volumes XVI, XVII, and XVIII of the Results of the National Argentine Observatory, 1892, 1894, and 1900.

The letters N.G.C. signify A New General Catalogue of Nebulae and Clusters of Stars, being the catalogue of the late Sir John F.W. Herschel, Bart., revised, corrected, and enlarged by Dr. J.I.E. Dreyer. This forms Volume XLIX, Part I, of the Memoirs of the Royal Astronomical Society, and was published in 1888. It was extended by Dreyer in two supplementary catalogues of objects discovered by later observers, known as Index Catalogue of Nebulae, found in the years 1895 to 1907, being Volume LIX of the Memoirs. These are designated in this Atlas as N.C.G. I and N.C.G. II.


Bibliography

A list of papers concerning the photography of the Milky Way, its star clouds, nebulosities, and dark objects, etc., is given below. The abbreviations are:

S.M. = Sidereal Messenger

A. and Ap. = Astronomy and Astrophysics

Ap.J. = Astrophysical Journal

A.J. = Astronomical Journal

P.A. = Popular Astronomy

M.N. = Monthly Notices of the Royal Astronomical Society

A.N. = Astronomische Nachrichten

"Small Black Hole in the Milky Way," A.N., 108, 369, 1884

"Stellar Photography," S.M., 6, 58, 1887

"The Cluster G.C. 1420 and the Nebula N.G.C 2237 [1 sketch]," A.N., 122, 253, 1889

"One Some Celestial Photographs Made with a Large Portrait Lens in the Lick Observatory in 1889," Publications of the Astronomical Society of the Pacific, 2, 240–244, 1890

"Photographing with a Non–Photographic Telescope," S.M., 10, 331, 1891. (Reprinted from Anthony's International Annual of Photography, 1891.)

"Photographic Nebulosities and Groups of Nebulous Stars," A.N., 130, 233, 1892

"Photographic Nebulosities and Star Clusters Connected with the Milky Way [1 plate with chart]," A. and Ap., 13, 177, 1894

"Photograph of Swift's Nebula in Monoceros [1 plate and chart]," ibid., p.642, 1894

"On the Exterior Nebulosities of the Pleiades [1 plate]," A.N., 136, 193, 1894. (Reprinted in A. and Ap., 13, 768, 1894.)

"Photographs of M8 and the Trifid Nebula [1 plate]," A. and Ap., 13, 791, 1894

"The Great Photographic Nebula of Orion Encircling the Belt and Theta Nebula [1 plate]," ibid., p. 811, 1894; P.A.., 2, 151, 1894–1895

"Photographs of the Milky Way [2 plates]," Ap.J., 1, 10, 1895

"Photographs of the Milky Way near 15 Monocerotis and near eCygni [2 plates]," ibid., 2, 58, 1895

"Photograph of the Nebula N.G.C. 1499 near the Star ?Persei [1 plate]," ibid., p. 350, 1895

"Celestial Photographs with a Magic Lantern Lens [1 plate]," ibid., pp. 351–353, 1895

"On a Great Photographic Nebula in Scorpio, near Antares [2 plates]," M.N., 55, 453, 1895; A.N.., 138, 211–215, 1895; P.A., 5, 227–232, 1897

"On the Extended Nebulosity about 15 Monocerotis," M.N., 56, 63, 1895

"On the Exterior Nebulosities of the Pleiades [1 plate]," Knowledge, 18, 282, 1895

"Extended Nebulosities near Antares [1 plate]," ibid., 19, 205, 1896

"On the Comparison of Reflector and Portrait Lens Photographs [1 plate]," M.N. 57, 10–16, 1896

"Astronomical Photography with Small Lenses,"Ap.J., 6, 446, 1897

"The Great Nebula of Rho Ophiuchi and the Smallness of the Stars Forming the Groundwork of the Milky Way [2 plates and chart]," P.A., 5, 227–232, 1897

"The Development of Photography in Astronomy," Proceedings of the American Association for the Advancement of Science, 47, 19–51, 1898. (Reprinted in P.A.., 6, 425–455, 1898.)

"Note on the Exterior Nebulosities of the Pleiades," M.N., 59, 155, 1899

"Photographs of Comets and the Milky Way [3 plates]," ibid., pp. 354–370, 1899

"Photographs of the Milky Way near the Star ? Ophiuchi [1 plate]," Ap.J., 9, 157–158, 1899

"The Exterior Nebulosities of the Pleiades, with a Drawing from the Different Photographs: and on the Appearance of the Cluster with the 40–inch Refractor [2 plates]," M.N., 60, 258–261, 1900

"Diffused Nebulosities in the Heavens," A.J>, 17, 77–80, 1903

"The Bruce Photographic Telescope of the Yerkes Observatory [4 plates]," Ap.J., 21, 35–48, 1905

"A Great Photographic Nebula near p and d Scorpii [1 plate]," ibid., 23, 144–147, 1906

"On the Vacant Regions of the Sky [3 plates]," P.A., 14, 579–583, 1906

"On A Nebulous Groundwork in the Constellation Taurus [2 plates]," Ap.J., 25, 218–225, 1907. (Reprinted in Bulletin de la Societe Belge d'Astronomie, 13, 129–136, 1908.)

"Some fo the Results of Astronomical Photography, Pertaining Specially to the Work with a Portrait Lens [6 plates]," Proceedings of the American Philosophical Society, 46, 417–429, 1907. (Reprinted in P.A., 16, 286–298, 1908.)

"Some Notes on Nebulae and Nebulosities," A.N., 177, 231–236, 1908

"Question of Absorbing Matter in Space and the Transparency of the Nebulae [2 plates and chart]," Ap.J., 31, 8–14, 1910

"Photographs of the Milky Way and of Comets [129 plates]," Publications of the Lick Observatory, 11, 1913

"Dark Regions in the Sky Suggesting an Obscuration of Light," Ap.J., 38, 496–501, 1913

"A Photographic Comparison of a Bright Nebula with a Dark One, and the Possible Luminosity of Space," P.A., 23, 596, 1915; Ap.J., 43, 1–8, 1916

"A Singular Dark Marking on the Sky," Proceedings of the National Academy of Sciences, 1, 394–396, 1915

"A Great Nebulous Region near Omicron Persei," M.N., 60, 261, 1900; Ap.J., 41, 253–258, 1915

"Some of the Dark Markings of the Sky and What They Suggest [2 plates]," Ap.J., 43, 1–8, 1916

"Some Remarkable Small Black Spots in the Milky Way," P.A., 26, 685, 1918

"On the Dark Markings of the Sky, with a Catalogue of 182 Such Objects," Ap.J., 49, 1–23, 360, 1919

"On the Comparative Distances of Certain Globular Clusters and the Star Clouds of the Milky Way," A.J., 33, 86, 1920

"Star Streams," Journal of the Royal Astronomical Society of Canada, 16, 51–53, 1922

Additional Brief papers referring to early photographic work on the Milky Way may be found as follows:

Sidereal Messenger, 2, 259, 1883

Knowledge, 13, 174, 186, 237, 1890; 14, 50, 93, 232, 1891; 15, 14–161, 1892; 16, 10–12, 1893; 17, 16, 17, 61, 181, 253, 1894

Astronomische Nachrichten, 126, 293, 1891

Biographical sketches of Professor Barnard by Edwin B. Frost have been printed in the Astrophysical Journal, 58, 1–35, 1923, and Memoirs of the National Academy of Sciences, 21, Fourteenth Memoir, 1926. A more complete Bibliography is given in the second named paper.

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