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 XLhc Taniversit? of Chicago 
 
 FOUNDED BY JOHN D ROCKEFELLFR 
 
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 THE YERKES OBSER V 
 
 WILLIAMS BAY, WISCONSInV> ^ 
 
 
 
 THE UNIVERSITY OF CHICAGO PRESS 
 CHICAGO, ILLINOIS 
 
Fig. I.—The Yerkes Observatory from the Northwest 
 
THE YERKES OBSERVATORY 
 
 By Edwin Brant Frost, Director 
 
 The establishment of a large observatory for astrophysical research in 
 connection with the University of Chicago was the conception of George E. 
 Hale, who had achieved success in his solar work with his private equipment in 
 Chicago, known as the Kenwood Observatory. President Harper took an 
 active interest in the realization of the plan, which presently took shape as an 
 opportunity occurred for the purchase of two very perfect glass disks (crown and 
 flint) of 42 inches diameter, sufficient for the construction of an object-glass 
 of 40 inches clear aperture. These disks had been cast by the well-known 
 makers of optical glass, Mantois of Paris, upon order of an institution in south¬ 
 ern California, which was, however, then unable to raise the money for figuring 
 and mounting the telescope. The opportunity was presented by Dr. Harper 
 and Mr. Hale to Mr. Charles T. Yerkes, a keen business man of Chicago, of 
 purchasing these disks and securing the largest refracting telescope in the world. 
 Mr. Yerkes agreed in September, 1892, to finance the undertaking, and a con¬ 
 tract was made with the famous firm of Alvan Clark & Sons, of Cambridge- 
 port. Mass., for figuring the two disks, which were then in their hands, and with 
 Warner & Swasey, of Cleveland, Ohio, for constructing a suitable mounting. 
 
 Mr. Hale was appointed director of the new institution, and gave much 
 study to the plans for the instruments and building. He outlined in October, 
 1892, the principal lines of scientific work which would be undertaken. 
 
 The site of the observatory was selected after a careful investigation of the 
 available locations and after an expression of opinion had been obtained from 
 numerous astronomers familiar with large telescopes. On account of the 
 smokiness, dust, and jar, as well as the glare due to the lights of a great city, it 
 was at once obvious that the observatory ought not to be established within or 
 near the limits of Chicago. But by reason of its connection with the Uni¬ 
 versity it was practically essential that it should not be placed more than one 
 hundred miles away from the other departments of the University. Tracts 
 of land were offered in more than twenty places in Illinois and elsewhere, but 
 the situation on Lake Geneva seemed to offer the advantages of freedom from 
 disturbance by commerce, present or prospective, and from the encroach¬ 
 ment of too numerous dwellings, together with sufficient accessibility from 
 the University and city. The site of 53 acres was presented by Mr. John 
 Johnston, Jr.; other adjacent land was later acquired by the Trustees of the 
 University, so that since 1907 the grounds have included about 70 acres, with 
 a frontage of 550 feet on the lake (where a pier for steamers is maintained in 
 summer). 
 
 3 
 
4 
 
 THE UNIVERSITY OF CHICAGO 
 
 Fig. 2.—The 40-iNCH Refractor, Taken with the Rising Floor Nearly at Its 
 
 Lowest Point 
 
 The Observatory is situated about 190 feet above the level of the lake, or 
 1,050 feet above the sea. Its geographical position, as determined by officers of 
 the United States Coast and Geodetic Survey in 1900, is: Latitude, 42° 34' 
 12^64; Longitude, 5^ 54“ 13^24 west of Greenwich. The grounds are one mile 
 from the post-office of the little village of Williams Bay, Wisconsin, and a 
 
THE YERKES OBSERVATORY 
 
 5 
 
 mile and a quarter from the station, at the terminus of a branch of the Chicago 
 & Northwestern Railway, 76 miles from Chicago.^ 
 
 In the summers of 1907 and 1908 a beginning was made in grading the 
 grounds of the Observatory after a plan designed by Olmsted Bros., of Brook¬ 
 line, Massachusetts. Further work in grading and planting was done in 1913. 
 When the original plan is completed and the planting has been carried out 
 as proposed, an appropriate setting will be secured for the architecturally 
 impressive building. 
 
 The Observatory was designed by Henry Ives Cobb, who carried out the 
 plans drawn up by Professor Hale in a way that successfully combined archi¬ 
 tectural effect with scientific utility. The large observatories of America and 
 Europe were visited by Mr. Hale before the plans were begun, the most useful 
 suggestions being received from the designs of the Lick Observatory and the 
 Astrophysical Observatory at Potsdam, Prussia. The style of the building is 
 Romanesque with rather elaborate details. It is constructed of brown Roman 
 brick, with terra-cotta ornaments to match. The shape is that of a Latin 
 cross, with the three towers and the meridian room at the extremities. The 
 long axis lies east and west, with the great tower at the west, 92 feet in diameter. 
 The entire length of the building in this direction is 326 feet. The two smaller 
 domes are on the north-and-south axis, with their centers 144 feet apart. 
 
 The basement floor contains the instrument shop, the machine shop, a car¬ 
 penter shop, a large physical laboratory adjoining a room fitted with a Rowland 
 concave grating, and several dark rooms for photographic work. The main 
 floor contains seven offices for the staff, a laboratory, computing room, and 
 lecture-room, a reception room for women, a library 40X20 feet, besides dark 
 rooms, chemical laboratory, and cabinets for instruments. The low attic is 
 used for a stackroom for books and for storage, and an important room called 
 the heliostat room, nearly 100 feet long, is provided on the second floor between 
 the smaller towers. At the north end the roof can be slid back so that a good 
 view of the sky is obtained, and a heliostat can be mounted on the pier at that 
 point. The horizontal beam of light can then be studied in the particular 
 manner desired in the long room, which is provided with numerous piers. 
 The most important piece of work thus far done in this laboratory was the 
 first successful measurement of the heat of the stars by Professor Ernest F. 
 Nichols (then of Dartmouth College) in the summers of 1898 and 1900. 
 
 THE GREAT REFRACTOR 
 
 The forty-inch telescope is mounted upon a massive brick pier, which rests 
 upon a solid concrete foundation set in the prevalent gravel formation. The 
 column is of cast-iron in four heavy sections. The center of motion of the 
 telescope is 62 feet above the ground. The length of the telescope is 62 feet, 
 
 •Additional railway facilities are obtained by the trolley line terminating at the head of the lake 
 (Fontana), two miles from the Observatory, which connects with the Chicago, Milwaukee & St. Paul 
 Railway (Chicago and Madison line) at Walworth, Wisconsin, and with the Chicago & Northwestern 
 Railway (main line to Minneapolis) at Harvard, Illinois, twelve miles distant. 
 
6 
 
 THE UNIVERSITY OF CHICAGO 
 
 and spectroscopic attachments can be added which increase the length by 
 nearly lo feet. The instrument can be readily moved by hand in spite of 
 the weight, 6 tons, of the tube, and that (total 20 tons) of all moving parts. 
 Electric motors are available, however, by which the instrument can be quickly 
 pointed toward any part of the sky. For spectroscopic and photographic work 
 it is necessary to correct the position of the telescope by very slight motions, 
 for which additional motors, controlled by the observer’s hand, are provided. 
 With these the eye-end can be moved by an amount as small as xio inch and 
 stopped at the position desired. The driving-clock, by which the telescope is 
 made to follow the stars, occupies a small room in the top of the pier. Its 
 performance has been remarkably satisfactory during the twenty-one years it has 
 now been in use, at temperatures ranging from —25° F. to +90° F., and under 
 great variations as to humidity. When the driving-weight reaches the end of 
 its descent within the pier, it touches a button, turning on a switch, and the 
 clock is wound up by a motor. By shifting a gear the rate of the clock may 
 be changed to make the telescope follow either the sun, or moon, or stars. 
 Finely graduated circles were provided for reading off the position of the 
 telescope, but in practice the two coarse circles, five and eight feet in diameter, 
 with large graduations visible from the floor, have been found sufficient and 
 quicker in operation. For sighting the instrument a finder of 4 inches aper¬ 
 ture is provided, and for the spectrograph a small finder of 60 feet focus, 
 without tube, is used. 
 
 The optical quality of the telescope is of the highest order; under favorable 
 atmospheric conditions very faint objects have been seen with it and close stars 
 separated. A German authority who had investigated the quality of many 
 objectives, by means of photographs taken inside and outside of the focus, 
 placed this at the head of the great lenses he had studied. A great telescope, 
 of long focal length, makes very exacting demands upon the atmosphere, far 
 more than is the case with a smaller instrument. Consequently a casual 
 visitor would very probably be disappointed not to find a higher magnification 
 employed on an average night. In a more favorable climate, such as that of 
 California, the performance would undoubtedly be much better than here.^ 
 In the work done with the spectrograph the steadiness of the air is not of as 
 much consequence, as unfavorable conditions merely protract the exposure 
 without injuring the quality of the plate. For visual observations of objects 
 which are sufficiently bright a large iris diaphragm has recently been con¬ 
 structed in the shop, by which any desired part of the whole aperture may be 
 used. 
 
 The operation of the large telescope is greatly facilitated by the admirable 
 arrangements of the dome and rising floor, designed and constructed by Warner 
 & Swasey. In order to have ample room for attaching spectroscopes and other 
 
 » Weather at Lake Geneva .—A record is kept of the number of hours during which the 40-inch tele¬ 
 scope is in use at night. It should be understood that except for cloudy skies it is employed throughout 
 every night of the year. In 1908, a very clear year, it was in use during over 1880 hours, or about 52 per 
 cent of the total night hours. It is doubtful if many European observatories could give as good a record 
 of clearness. 
 
THE YERKES OBSERVATORY 
 
 7 
 
 apparatus, it was necessary that the dome should be large, and its diameter is 
 90 feet. It is turned on 36 wheels by an electric motor which actuates an end¬ 
 less wire rope extending around the whole dome. The controller is on the 
 rising floor just at the north side of the pier. Six minutes are required for 
 turning the dome completely around. The rising floor is 75 feet in diameter, 
 and is supported by four cables running over four sheaves just under the upper 
 balcony and connecting with heavy counterpoises which balance a large part 
 of the weight of the floor (37I tons). A powerful motor, with its controller 
 conveniently near to that for turning the dome, supplies the power. The 
 floor moves through a range of 23 feet from the lower to the upper balcony. 
 It has to be nearly at its lowest point when an observer is looking at a star near 
 the zenith. In intermediate positions the floor can be quickly adapted to 
 the height of the eyepiece of the telescope. The opening in the dome through 
 which the sky is observed is ii feet wide, and it is closed by shutters 85 feet 
 long. These are so well constructed that they can be very easily moved by 
 hand. Canvas screens, which may be raised over part of the opening, serve 
 to break the force of the wind. 
 
 ACCESSORIES AND LINES OF RESEARCH 
 
 The principal accessories of the telescope and the lines of work done with 
 each may now be mentioned. The filar micrometer is attached at the eye-end 
 for nearly all visual observations. It was presented by Mr. Yerkes and con¬ 
 structed by Warner & Swasey. It enables the observer to determine very 
 exactly the angular separation of two objects (or points) both of which are 
 visible in the eyepiece at the same time, and to fix the angle made by the line 
 joining the points with the north-and-south direction. The micrometer has 
 during the past twenty years been in use for a little more than one-half the time, 
 generally on three or four nights per week. With it Mr. Burnham made a 
 great number of observations, most of which were utilized in his monumental 
 catalogue of double stars published in 1906. Mr. Barnard has similarly meas¬ 
 ured with great skill and persistence the positions of many stars in a number of 
 the star clusters, of the fainter satellites of our system, of nebulae and faint 
 comets, and has observed the most important objects suitable for visual 
 examination. Double stars are now observed chiefly by Mr. Van Biesbroeck. 
 
 A very important method of using the telescope as a camera was successfully 
 applied by Mr. G. W. Ritchey, principally in 1900 and 1901. The photographs 
 obtained by him of the moon, star clusters, and some nebulae were at once 
 recognized as among the very best thus far secured. For this service there is 
 substituted, in place of the eyepiece, a plate-holder, which can be very delicately 
 moved either north and south or east and west, and can thus follow the slight 
 oscillations of the object due to the unsteadiness of our atmosphere during the 
 exposure. The crown and flint lenses of the 40-inch objective were of course 
 so figured as to be achromatic for the rays to which the eye is most sensitive, 
 the yellow and the green. By the interposition of a color-filter cutting down 
 
8 
 
 THE UNIVERSITY OF CHICAGO 
 
 Fig. 3.—Lunar Crater Theophilus and Surrounding Region 
 
 Photographed with the 40-inch refractor in less than one-half second (Ritchey). 
 The diameter of the large crater is 64 miles; its walls rise to a height of 18,000 feet, 
 and the central peaks (notice their shadows cast toward the left) are about 6,000 
 feet high. 
 
 the blue and the violet rays, which are not simultaneously in focus with the 
 visual rays, and by the use of isochromatic plates, which are sensitized for the 
 visual rays, a sharp photograph is obtained. A negative of a star cluster or 
 other object, secured in this way when atmospheric conditions are particularly 
 good, is then measured under a microscope in the laboratory, and even weeks 
 of work can be profitably spent upon a single fine negative. 
 
 During 1904 and 1905 a large number of stellar photographs were thus 
 obtained by Mr. Schlesinger, who measured them most carefully in order to 
 determine from the plates the distances from the sun of certain selected stars. 
 The very accurate results were published in 1911. This work has been con¬ 
 tinued by Messrs. Slocum, Lee, Van Biesbroeck, and others. 
 
THE YERKES OBSERVATORY 
 
 9 
 
 Fig. 4.—Mars 
 
 Photographs made on September 28, 1909, with 40-inch telescope (Barnard). 
 
 A small lens takes the place of the eyepiece and enlarges the image about three 
 times. Numerous exposures of 3 or 4 seconds’ duration are made in quick succession 
 in order to catch a moment when the atmosphere permits good definition. 
 
 Note the bright spot at the left of the top of the pictures: this is the south polar 
 “cap,” probably due to snow in the planet’s polar regions; it diminishes rapidly as the 
 planet’s summer comes on for that hemisphere. 
 
 It is not known what the large dark regions represent. They were formerly 
 supposed to be oceans, but this idea is now abandoned. It is thought that there are 
 no large bodies of water on the planet. The lighter regions are possibly deserts, and 
 the dark ones are possibly due to vegetation. 
 
lO 
 
 THE UNIVERSITY OF CHICAGO 
 
 Another attachment of the telescope is a stellar photometer with which the 
 brightness of the stars is measured. Observations of many faint stars have 
 thus been made by Mr. Parkhurst, both visually and photographically. 
 
 About one-fourth of the nocturnal hours of the telescope, generally two 
 nights per week, are devoted to stellar spectroscopy. The instrument, which 
 was largely constructed in the shops of the Observatory, is designated as the 
 Bruce spectrograph, the funds for its construction and early operation having 
 been contributed by the late Miss Catherine W. Bruce, of New York. A 
 photograph of the spectrum of a star, accompanied by a comparison spectrum 
 on the same plate from metallic terminals vaporized by the passage of sparks 
 from an induction coil in the dome, can be measured under a microscope, 
 and the speed of the star in the line of sight (on the average about lo miles per 
 second) can be inferred from the displacements of the star’s lines from the 
 position of the lines due to the spark. About 8,000 photographs, or spectro¬ 
 grams, have been obtained to date with the instrument, chiefly by Messrs. 
 Frost, Adams, Barrett, and Lee. Interesting results have come from the 
 measurements of the spectrograms, and some 200 spectroscopic binaries, or 
 double stars so close to each other that they can be separated only in this 
 spectroscopic manner, have been detected. 
 
 An especially important use of the great refractor has been in the study of 
 the sun. The attachment generally used is the spectroheliograph, so named by 
 Mr. Hale, its inventor, when he brought it into successful operation at the Ken¬ 
 wood Observatory. 
 
 The instrument regularly in use here since 1903 is known as the Rumford 
 spectroheliograph. It is a heavy spectrograph (700 lbs.) with which, by the 
 motion of the plate while the sun’s image is passing across the slit, pictures of 
 the sun’s surface and surrounding prominences may be secured in monochro¬ 
 matic light, from some one line of calcium, or hydrogen, or other element which 
 may be particularly active on the sun. Adjustments can be so made that the 
 photograph will depict either the lower, the intermediate, or the upper levels 
 of the incandescent vapors that lie above the surface. The scarlet radiation of 
 hydrogen shows in a marked degree the vortices forming around a sun-spot 
 (which may be regarded as a storm in the sun’s atmosphere), as was first found 
 by Mr. Hale with his fine apparatus installed on Mount Wilson, California. 
 Visual and photographic studies of the sun are made with other spectroscopes, 
 and direct photographs are also taken with the great telescope. This work 
 was carried on by Messrs. Hale and Ellerman in 1903-4; then by Mr. Fox; 
 from 1909 to 1914 by Mr. Slocum, and subsequently by Mr. Lee and others. 
 
 OTHER TELESCOPES AND APPARATUS 
 
 The southeast dome, 30 feet in diameter, contains the two-foot reflector, an 
 instrument quite as powerful for some purposes as the 40-inch telescope itself. 
 The silvered concave mirror was made in the optical shop by Mr. Ritchey, who 
 also supervised the construction of the mounting in the Observatory machine 
 
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 THE YERKES OBSERVATORY 
 
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12 
 
 THE UNIVERSITY OF CHICAGO 
 
 Fig. 6.—The Sun, Photographed with the Spectroheliograph on 
 August 12, 1903 (Hale and Ellerman) 
 
 The white blotches are areas of intensely brilliant calcium vapor, which would 
 be invisible on an ordinary photograph. 
 
 shop. The figure of the mirror is very perfect, and remarkably fine photo¬ 
 graphs of nebulae have been obtained with it, principally by Mr. Ritchey. 
 It has the great advantage over a refractor that the rays of all colors are united 
 in the same focus, here 8 feet from the surface of the mirror. A star of the 
 ninth magnitude (having less than one-thousandth of the brightness of a first- 
 magnitude star) can be photographed with it in one second, and the instru¬ 
 ment is exclusively used photographically. With an exposure of three hours 
 vast numbers of stars will impress their images on the plate, many of which 
 cannot be seen at all with the 40-inch refractor. Some work on stellar spectra 
 has also been done with the instrument. 
 
 The northeast dome contains the Kenwood equatorial refractor of 12 inches 
 aperture, which, together with much valuable equipment, was given to the 
 University by Mr. William E. Hale and Mr. George E. Hale, and moved here 
 
THE YERKES OBSERVATORY 
 
 13 
 
 Fig. 7,—Composite or Two Plates Made on August 14, 1907 (Fox) 
 Showing calcium vapor as prominences at sun’s edge and on the disk 
 
 from its former location at their residence in Chicago. The instrument has 
 two objectives, one for visual work, the other for photography, both made by 
 Mr. J. A. Brashear. The mounting is by Warner & Swasey. An excellent filar 
 micrometer, by the last-named firm, was purchased for this telescope in 1906. 
 The old stellar spectrograph, provided by Mr. Yerkes in the original equipment 
 for the 40-inch telescope, has recently been adapted for use in solar spectroscopy 
 with the 12-inch telescope. The telescope has been used for observations of 
 comets, double stars, variable stars, for direct photographs of the sun, stellar 
 photometry, etc. A modern twin mounting is urgently needed. 
 
 On the main roof, between the two small domes, is housed the Brashear 
 comet-seeker, of six inches aperture. A small camera is also installed here to 
 serve as sky-patrol. In the meridian room at the east end of the building is 
 located the broken-tube transit of 3 inches aperture, made in the machine shop. 
 
 At the north window of the short corridor leading to the meridian room are 
 the barometer, maximum and minimum thermometers, and the thermograph. 
 Observations are made twice a day. A thermograph is installed on the north 
 side of the great dome on the outside balcony. A shelter, of the United States 
 Weather Bureau’s pattern, containing maximum and minimum thermometers, 
 was placed on the lawn in 1917. No seismographs for recording earthquakes, 
 nor instruments for determining the state of the earth’s magnetism, are main¬ 
 tained at the Observatory. 
 
14 
 
 THE UNIVERSITY OF CHICAGO 
 
 In an upper room has been set up an apparatus devised by Professor Hale 
 by which the photographs of the sun taken with the spectroheUograph can be 
 optically projected upon a steel globe ruled with parallels and meridians, so 
 that the positions of significant markings on the sun can be easily read off 
 without trigonometrical computation. 
 
 An important adjunct of an astrophysical observatory is its spectroscopic 
 laboratory, where experiments may be made to match in some degree the con¬ 
 ditions found in celestial objects, in running to earth unknown elements in their 
 spectra, and in various researches calculated to assist in the interpretation of the 
 phenomena of the heavenly bodies. Unfortunately it has not been possible to 
 obtain funds for keeping a physicist steadily employed. During two or three 
 years, principally from a gift of about $4,000 made by Mr. Yerkes, such work 
 was in progress. Extensive studies were made by Mr. N. A. Kent, in collabora¬ 
 tion with IMr. Hale, on the effect of heavy pressure upon the spectrum of the 
 spark; by IMr. Kent on the comparison of w^ave-length in the spectnun of the 
 same element when rendered luminous in the spark and in the arc, and by 
 Mr. H. M. Reese on the enhanced lines in spark spectra. A large spectrome¬ 
 ter by Fuess has recently been added to the equipment of this laboratory. 
 
 In view of the very general use of photography in modern astronomy, a 
 laboratory for photographic research is important for an institution like this. 
 In the years from 1903 to the end of 1908 such a laboratory was in successful 
 operation under the charge of Mr. R. J. Wallace. Particularly during the 
 years 1905-8, under a grant to the Observatory from Mr. George Eastman, of 
 Rochester, New York, Mr. Wallace was able to devote his time to research on 
 various problems in photography, including the adaptation and sensitization 
 of plates for particular purposes. Aside from the improvement of the photo¬ 
 graphic routine in the different departments of the Observatory, these investi¬ 
 gations resulted in important papers published in the Astrophysical Journal 
 and elsewhere. 
 
 The great optical works at Jena, having the firm-name Carl Zeiss, have 
 rendered great service in the development of microscopes and apparatus for 
 measuring photographs. Several are owned by the Observatory, the most 
 important being known as the “Stereo-comparator” and “Blink-mikroskop,” 
 purchased in 1907. This is a massive instrument in which two different nega¬ 
 tives can be examined, either at the same time or in instantaneous succession, 
 so that any change in position, size, or intensity of the images is at once notice¬ 
 able to the eye, and discoveries are greatly facilitated. Accurate measure¬ 
 ments of various kinds can also be made with this apparatus. Another valu¬ 
 able accessory is the Hartmann spectro-comparator, acquired at ihe same time, 
 with which two negatives of spectra can be simultaneously compared and 
 measured against each other. Several other measuring machines, some of 
 them made in the Observatory shop, are in use for the precise study of plates. 
 In the computing room, calculating machines made in Europe, capable of 
 handling 18 figures, are principally employed for multiplication and division. 
 
THE YERKES OBSERVATORY 
 
 15 
 
 Fig. 8.—The 24-lNCH Reflector, Occupying the Southeastern Dome 
 
 The silvered glass mirror was made by Mr. Ritchey in the optical shop, the 
 mounting in the machine shop, of the Observatory. It will photograph stars which 
 cannot be seen with the great refractor. 
 
i6 
 
 THE UNIVERSITY OF CHICAGO 
 
 Fig. 9.—The Great Nebula in Orion, Photographed with the 24-lNCH 
 Reflector (Ritchey) 
 
 This inconceivably large mass of gas, chiefly hydrogen, helium, and nebulum 
 (not yet found in the earth), is a feature of the constellation Orion. 
 
 As the original equipment of the Observatory was chiefly limited to the 
 40-inch and 12-inch telescopes, much remained to be done in fitting out the 
 institution for its best efiflciency. The Trustees of the University adopted 
 the important recommendation of the Director and provided from the start 
 for maintaining an instrument-maker and machine shop, the machines of 
 which were chiefly donated from the Kenwood Observatory. In this way the 
 equipment has been gradually increased, kept in repair, and perfected. At 
 times, when special gifts were available, three men have been employed in the 
 shop. The principal apparatus constructed have been these: the two-foot 
 reflector, the Snow horizontal telescope, the Bruce spectrograph, the transit 
 instrument, a spectrohehograph, a coelostat for eclipse use, a quartz spectro¬ 
 graph for the two-foot reflector, and many smaller instruments and attach¬ 
 ments. In the carpenter shop a great number of patterns have been made, and 
 
THE YERKES OBSERVATORY 
 
 17 
 
 Fig. 10. —Nebula in Triangulum (Messier 33), Photographed with the 24-lNCH 
 
 Reflector (Ritchey) 
 
 The spiral structure can readily be seen. Its distance from the earth is not 
 less than that of the surrounding stars, which, it should be recalled, are all suns, pre¬ 
 sumably not smaller and perhaps much larger than our own sun. 
 
i8 THE UNIVERSITY OF CHICAGO 
 
 special cabinets of various kinds have been constructed for negatives, charts, 
 and other valuable records. 
 
 HORIZONTAL TELESCOPE 
 
 Owing to the great convenience of operation of a horizontal coelostat tele¬ 
 scope for solar work where massive spectroheliographs must be attached and 
 removed, Mr. Hale early began the construction of such an instrument. The 
 first one had barely been completed, in a temporary wooden and paper struc¬ 
 ture outside of the Observatory, when it was burned (December 22, 1902) from 
 the sparks of the high-potential apparatus used for the comparison spectrum. 
 A 30-inch plane mirror and a 24-inch concave mirror, made in the optical shop, 
 were destroyed, together with the driving-clock and all but the heavy castings 
 of the coelostat. A few months later Miss Helen M. Snow, of Chicago, made a 
 gift of $10,000 in honor of her father, the late George W. Snow, for reconstruct¬ 
 ing the instrument. A wooden building was erected 600 feet north of the 
 Observatory, and by great activity in the optical and machine shops it was 
 possible formally to open the Snow telescope in October, 1903. 
 
 Having been interested in the possibilities of solar research under the more 
 perfect atmospheric conditions of southern California, Mr. Hale, in the next year, 
 obtained a grant of $10,000 from the Carnegie Institution of Washington for an 
 expedition for solar research from the Yerkes Observatory to Mount Wilson, 
 California. The Snow telescope was safely transported thither and mounted 
 in a steel and canvas building, which experience showed to be more suitable 
 than the wooden structure. The results obtained were so satisfactory and so 
 promising for the future that the Carnegie Institution decided to establish 
 its own observatory there. An arrangement satisfactory to the donor and all 
 others interested was later made whereby the Snow telescope was purchased 
 by the Solar Observatory, the money thus returned to the University of Chicago 
 being used to buy the stereo-comparator, and $5,000 being established as the 
 Snow fund, from the interest of which instrumental equipment can be pur¬ 
 chased. This was the first endowment fund provided specifically for the use 
 of the Observatory. In this connection it may be stated that in his will Mr. 
 Yerkes made a bequest for the maintenance of the Observatory of $100,000, 
 a portion of which was received from his estate in 1913. 
 
 In 1908 the eclipse coelostat was mounted in the Snow building in con¬ 
 nection with an autocollimating spectrograph, and a powerful instrument for 
 research in the solar spectrum was secured at small additional cost. 
 
 BRUCE PHOTOGRAPHIC TELESCOPE 
 
 In 1897, in response to a presentation of the advantages of such an instru¬ 
 ment by Mr. Barnard, Miss Catherine W. Bruce, of New York, placed in his 
 hands, as a gift to the University of Chicago, the sum of $7,000 for the con¬ 
 struction of a telescope especially designed for the photography of the large 
 areas and extended objects in the sky, particularly the Milky Way and comets— 
 a field in which Mr. Barnard has been a particularly successful pioneer. A 
 
THE YERKES OBSERVATORY 
 
 19 
 
 number of years were spent in the endeavor to secure a lens which should fully 
 meet the ideal requirements. A fine lo-inch doublet, constructed by Brashear, 
 was finally accepted, in 1900, and a special mounting was designed and built 
 by Warner & Swasey. The instrument was set up in April, 1904, in a neat 
 wooden building (erected from interest on the Bruce fund), 350 feet southwest 
 of the great dome of the Observatory. In addition to the lo-inch (focal 
 length, 50 inches) the mounting also carries a 6|-inch Voigtlander doublet, a 
 5-inch visual guiding telescope, and generally a 3|-inch portrait lens. Many 
 photographs of the Milky Way, showing good definition over a field of 50 
 square degrees, have been obtained by Mr. Barnard. In 1905 he spent nine 
 months at Mount Wilson (altitude nearly 6,000 feet) in photographing the Milky 
 Way, particularly those southern portions not to be reached at Lake Geneva. 
 This trip was made possible by a grant from Mr. J. D. Hooker, of Los Angeles. 
 The Carnegie Institution of Washington has lately made a grant for the purpose 
 of publishing an atlas of reproductions of these pictures of the Milky Way, 
 and the work is now in progress. Of the comet Morehouse, discovered on 
 plates taken with the Bruce telescope by Mr. Morehouse on September i, 
 1908, Mr. Barnard secured no less than 350 negatives. 
 
 ZEISS ULTRA-VIOLET CAMERA 
 
 Another photographic lens of special value is the Zeiss doublet, made of 
 glass particularly transparent to ultra-violet rays, of 5.7 inches aperture and 
 32 inches focal length. The stellar images it produced when slightly out of 
 focus were found to be disks of remarkably uniform intensity. By measuring 
 the intensity of these disks under the Hartmann “surface photometer” Mr. 
 Parkhurst has attained an exceedingly high degree of accuracy in stellar 
 photometry. Photographs taken with an exposure of twenty minutes, with 
 wire gratings placed over the objective, give measurable focal images for 
 photometric purposes of thirteenth-magnitude stars. 
 
 Objective-prisms of the same transparent glass and with the same aperture 
 as the lens, with refracting angles of 15° and 30°, respectively, are provided 
 for the doublet. They make possible the study of the spectra of all the stars 
 which are within the field of view.|^This apparatus has proved to be particularly 
 useful for photographing the spectra of comets. 
 
 This important instrument has heretofore been used provisionally on a 
 mounting belonging to Mr. Parkhurst and in a temporary building located 
 1,000 feet north of the Observatory. It will be transferred to a new small 
 dome to be erected on the Observatory grounds northeast of the main building 
 as soon as the necessary funds can be secured. 
 
 The power-house, which also furnishes the steam heat (largely exhaust) 
 for the main building, is situated about 750 feet northeast from the center of 
 the large dome. It was included in the original gift of Mr. Yerkes. Its equip¬ 
 ment consists of duplicate boilers, and an^SXio Ideal engine, directly con¬ 
 nected with a Siemens & Halske dynamo, of capacity 200 amperes at 125 
 volts, which furnishes current Jor the motors in the shops, for the great 
 
20 
 
 THE UNIVERSITY OF CHICAGO 
 
 Fig. II.—The Bruce Photographic Telescope as Temporarily Set up in the 
 Corridor of the Yerkes Observatory 
 
 It is now located in the small dome to the southwest of the Observatory. 
 With this Professor Barnard has obtained many unsurpassed photographs of the 
 Milky Way and of comets. 
 
THE YERKES OBSERVATORY 
 
 21 
 
 Fig. 12. —A Small Portion of the Milky Way in Cepheus, Photographed with 
 THE Bruce Telescope on September ii, 1904 (Barnard) 
 
 Exposure 6 Hours and 40 Minutes 
 
 As will be seen in this picture, the Milky Way consists of innumerable faint stars 
 (in reality distant suns) separately indistinguishable with the naked eye, which are 
 distributed irregularly in the sky. Only three or four of the stars on this plate can 
 be seen with the naked eye. 
 
22 
 
 THE UNIVERSITY OF CHICAGO 
 
 dome and rising floor, for the 30-foot dome, and for all the general lighting 
 and dark-room lamps of the Observatory. The water for all purposes of the 
 institution is obtained from the lake by an electric pump at the shore operated 
 by current from the power-house, and is stored in a 30,000-gallon tank near 
 the power-house. 
 
 Questions are so often raised as to the cost of the equipment that a brief 
 statement may be given here. The land originally given was valued at $30,000. 
 The cost of the completed object-glass of the great refractor was $66,000; of 
 the telescope mounting itself, $55,000; of the dome and rising floor, $45,000, 
 and of the remainder of the Observatory building, including the southeast 
 dome and the power-house and its equipment, about $150,000. The instru¬ 
 ments and equipment of the Kenwood Observatory, with its dome, the gift 
 of Mr. William E. Hale and his son, were valued at $30,000. The Bruce 
 telescope and building cost about $8,000; the Snow telescope and building, 
 $10,000. About $15,000 have thus far been spent on grading and planting. 
 
 In response to the numerous calls for lantern slides, transparencies, and 
 prints from astronomical negatives made at this Observatory, it became 
 necessary several years ago to arrange for supplying them to teachers, lecturers, 
 and others interested. A photographer is employed in preparing these repro¬ 
 ductions, which are sold at a price to cover their cost, through the medium 
 of the University of Chicago Press. A catalogue of such slides, etc., including 
 a total of about 800 subjects, has been printed, and is sent on request by the 
 University of Chicago Press. 
 
 PUBLICATIONS 
 
 Current results of investigations at the Observatory are published in various 
 journals, American and foreign. Those of an astrophysical character gener¬ 
 ally appear in the Astrophysical Journal, of which the director of the Observa¬ 
 tory is the editor, published ten times a year by the University of Chicago 
 Press. The Carnegie Institution of Washington has published several separate 
 volumes by members of the staff, two of which have already been referred to. 
 Others are: Researches in Stellar Photometry by J. A. Parkhurst; The Rotation 
 Period of the Sun by George E. Hale and Philip Fox; Measures of Proper 
 Motion Stars by S. W. Burnham. Prior to 1916 the University had not been 
 able to provide a regular fund for the issue of the formal quarto Publications 
 of the Yerkes Observatory, and these are therefore much in arrears. The 
 volumes so far issued are: I (1900), pp. 296, A General Catalogue of i,2go 
 Double Stars Discovered from i 8 yi to i 8 gg, by S. W. Burnham. H (1904), 
 pp. 413, papers by Messrs. Burnham; Barnard; F. R. Moulton; Frost and 
 Adams; Hale, Ellerman, and Parkhurst; Ritchey; and Laves. HI, Parts i 
 and 2 (1903), by Hale and Ellerman, and Hale and Kent. IV, Part i (1917), by 
 Slocum, Mitchell, Lee, Joy, and VanBiesbroeck; Part 2 (1918), by E. P. Hubble; 
 Part 3 (1920), by Van Biesbroeck and ]\lrs. Pettit. Part 3 of Vol. HI, by 
 Fox, is in press (1921). 
 
THE YERKES OBSERVATORY 
 
 23 
 
 Fig. 13.— Halley’s Comet as Photographed with Bruce Telescope 
 ON May 29, 1910 (Barnard) 
 
 The plate was exposed for two hours. The observer kept the telescope constantly 
 pointed on the comet, following its motion among the stars, which caused the elonga¬ 
 tion of the star images. The tail of the comet, as shown on this plate, was about 
 7 million miles long. It was composed ot luminous vapors, chiefly hydrocarbons. 
 
3 0112105737750 
 
 24 THE UNIVERSITY OF CHICAGO 
 
 INSTRUCTION 
 
 Undergraduate instruction in astronomy is not given at the Observatory. 
 This is provided at the University, together with thorough courses in theoretical 
 astronomy and celestial mechanics taught by Professors Moulton, Laves, and 
 MacMillan. Graduate students competent in observational work in astron¬ 
 omy and astrophysics are welcome and may become Fellows under the usual 
 University regulations. All candidates for the Doctor’s degree in the depart¬ 
 ment are required to work at the Observatory at least for one quarter. 
 Astronomers from other institutions often come as volunteer research assist¬ 
 ants. The Astronomical Club meets frequently for discussions. 
 
 The Observatory library contains at present over 6,ooo volumes and 5,000 
 pamphlets. About 80 scientific magazines and journals are regularly received, 
 principally in exchange for the Astrophysical Journal. 
 
 The pressure for time for their scientific use has made it impossible to per¬ 
 mit visitors to see through the telescopes. Opportunity is given, however, 
 for them to inspect the Observatory and the great refractor, on Saturdays: 
 from June i to September 30 between i: 30 and 4:30 p.m. ; during the remainder 
 of the year, between 10 and 12 a.m. A member of the staff demonstrates the 
 operation of the large telescope and explains the work of the Observatory. 
 Interesting astronomical photographs are displayed in the corridors. Several 
 thousand visitors annually avail themselves of this opportunity. 
 
 The staff of the Observatory is constituted as follows (1921-22): 
 
 Edwin B. Frost, Professor of Astrophysics, and Director. 
 
 Edward E. Barnard, Professor of Practical Astronomy. 
 
 John A. Parkhurst, Associate Professor of Practical Astronomy. 
 
 Storrs B. Barrett, Assistant Professor of Astrophysics, Secretary, Librarian. 
 George Van Biesbroeck, Assistant Professor of Practical Astronomy. 
 Oliver J. Lee, Instructor in Practical Astronomy. 
 
 J. Paraskevopoulos (of Athens), Volunteer Research Assistant (1919-21). 
 Ernest C. Bryant (Middlebury), Clifford C. Crump (Ohio Wesleyan), 
 Charles E. Rogers (Trinity), Volunteer Research Assistants (Summer 
 1921). 
 
 Otto Struve,. Assistant in Stellar Spectroscopy. 
 
 Mary R. Calvert, Computer. 
 
 Florence B. Lee, Office Secretary. 
 
 Harriet McW. Parsons, Fellow (1920-21). 
 
 Elsie Johnson, Lela Cable, Esther Searles, Temporary Computers. 
 George C. Blakslee, Photographer. 
 
 Frank R. Sullivan, Engineer in charge of 40-inch telescope. 
 
 Stephen A. Stam, Instrument Maker. 
 
 Henry M. Foote, Carpenter, and Supervisor of Building. 
 
 Diedrich j. Oetjen, Engineer at Power House. 
 
 Wm. Koeppen, Night Engineer at Power House. 
 
 Carl I. Wendell, Janitor. 
 
 Carl Bertelson, Gardener.