362 CARNEGIE INSTITUTION OF WASHINGTON. 



As the light undergoes three reflections in each telescope, and as the 

 small mirrors cut out nearly the same proportion of light in both cases, 

 the theoretical ratio of light-gathering power would be ( VV )^ = 2.78. The 

 presence of the plane mirror in the optical train of the spectrograph of 

 the 60-inch telescope, and the fact that the three mirrors of the 100- 

 inch telescope have been more recently silvered than those of the 60- 

 inch, probably accounts in part for the comparatively high value of the 

 ratio at X 4000 (4.5), as the comparison spectra show no such change of 

 relative intensity with wave-length. But these differences can not 

 affect the brightness at X 4500 by more than 15 per cent, judging from 

 the constancy of the exposure-times for stars of a given magnitude 

 required with the 60-inch throughout the dry summer season. An- 

 other comparison will be made after the mirrors of the 60-inch have 

 been resilvered. 



A second comparative test, not based upon simultaneous exposures, 

 is afforded by Mr. Merrill's experience with stars of class Md. 201 of 

 these interesting objects, brighter than magnitude 9.0 at maximum, are 

 known north of —30°. With the 60-inch nearly all of these stars can 

 be observed for the bright lines with exposures not exceeding 2 hours. 

 For the great majority, however, exposures of 5 hours or more are 

 required with this telescope to yield a measurable absorption spectrum 

 with the spectrograph already mentioned. In fact, so few stars can 

 be effectively observed for both dark and bright lines that it would 

 be hardly advisable to enter upon an extensive study of these objects 

 with the 60-inch. 



The greater light-gathering power of the 100-inch, however, renders 

 such a study perfectly feasible. With this telescope, using the spectro- 

 graph mentioned above at the 134-foot Cassegrain focus, Mr. Merrill 

 has obtained good photographs of the absorption spectrum of RY 

 Herculis, visual magnitude 8.8, in 2 hours, and of brighter stars of the 

 same class with shorter exposures. Mr. Joy has experienced an equal 

 advantage over the 60-inch in photographing the spectra of stars of 

 other types. 



Mr. Shapley, who is continuing his investigation of star-clusters with 

 the 100-inch telescope, finds a similar gain of about one magnitude. 

 With the small slitless spectrograph mounted on the double-slide 

 plate-holder at the 134-foot focus of the 100-inch, the exposure-times 

 for stars in the globular cluster Messier 1 1 are about as follows : 



Photographic magnitude, 12; Exposure, 5 minutes. 

 13; " 15 



A further advantage results from the great scale of the clusters in 

 this focus of the 100-inch, which permits the spectra of closer stars to 

 be photographed separately. 



Mr. Pease has obtained some excellent direct photographs of the 

 stars in the central part of the globular cluster Messier 13 at the 



