230 



CARNEGIE INSTITUTION OF WASHINGTON. 



COMPARATIVE TESTS OF THE 100-INCH AND 60-INCH REFLECTORS. 



Last year's report gives a detailed account of comparative tests of 

 the large reflectors when used for the photography of stellar spectra. 

 Further experience confirms these results, w^hich show that the three- 

 fold light-gathering power of the 100-inch reflector over that of the 60- 

 inch is very nearly utilized to its full extent. A plane mirror used in 

 the optical train of the spectrograph on the 60-inch reflector is elimin- 

 ated from that of the 100-inch, w^hich secures a small gain of light for 

 the latter. With allow^ance for this fact, it is found that the relative 

 exposure-times average about 1 to 2.5 for the same stars. This mean 

 result refers to the same slit-width and the same length of camera, but 

 includes a wide range in the seeing and in the figure of the mirrors. 



To test the two instmments for limiting magnitude and resolution, 

 several series of photographs have been made under identical conditions 



