SOI.AR OBSERVATORY. 1 73 



ground. In this case the Hght is reflected vertically downward by means 

 of a small plane mirror, supported below the end of the polar axis. 



This second arrangement has been used for the experiments so far accom- 

 plished. The spectrograph, which is of the Littrow form, has an aperture 

 of 6 inches and a focal length of i8 feet. The electric slow motions of the 

 6o-inch reflector are under the control of the observer, who watches the star 

 image reflected from the polished slit jaws. As these slow motions work 

 very perfectly, and as the convex mirror can also be focused by a small 

 electric motor operated from the same point, it is easy to keep the star 

 image on the slit throughout the exposure. 



In the first experiments, which have been made by Mr. Adams, Mr. Bab- 

 cock and myself, the spectra of Arcturus and Antares have been photo- 

 graphed with a 64° dense flint prism, twice traversed by the light. This 

 prism, though the largest available, utilizes only a part of the beam. A 

 liquid prism, having faces 12 inches in diameter, is being tested in the hope 

 that it may serve for this purpose. 



In spite of unfavorable conditions, which can be avoided in later work, 

 some of the photographs are of fair quality. A discussion of the results 

 must be reserved until the work of measurement has been completed. It is 

 evident, however, that the results will give a first determination of the pres- 

 sure in the atmosphere of Arcturus. 



The second spectrograph constructed for the 60-inch reflector is designed 

 for use with the Newtonian combination, in the principal focus of the large 

 mirror. By means of a plane mirror, the diverging rays beyond the focal 

 plane are reflected toward the lower end of the telescope tube. They then 

 encounter a Voigtlander portrait lens of 5 inches aperture and 15.7 inches 

 focal length, which serves as a collimator. In this way the spectra of all 

 stars lying within a field about 30' in diameter can be photographed at once. 

 As it may be difiicult to get thoroughly satisfactory results without a slit, I 

 have devised a multiple slit, by means of which the spectra of from 6 to 10 

 stars can be photographed simultaneously. With the aid of comparison 

 spectra, it is hoped that the approximate radial motions and the spectral 

 classification of the tenth and eleventh magnitude stars in the central part of 

 Kapteyn's Selected Areas can be determined. The spectrograph is also pro- 

 vided with a spectroheliograph attachment, to be used in photographing 

 comets and nebulae with monochromatic light. 



This spectrograph has been employed by Mr. Adams and Dr. Fath in 

 photographing the spectra of star clusters and also of individual stars. The 

 preliminary tests of the multiple-slit method are sufficiently encouraging to 

 render advisable the construction of such a slit. 



A temporary wooden spectrograph was employed with the 6o-inch reflector 

 during the winter by Mr. Adams and Mr. Babcock in various preliminary 

 experiments. 



