SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 93 



was used by Messrs. Abbot, Stebbins, and Aldrich on Mount Wilson 

 in 1934 to measure the distribution of radiation in the spectra of stars 

 at the Conde focus of the lOO-inch reflector. The filters were mounted 

 within a constant-temperature box upon a squirrel-cage device, so as 

 to be successively introduced into a collimated beam. The selected ray 

 was brought to focus with a 19-cm focus lens. All this part of the 

 experiment worked well, and owing to the short-focus objective lens 



ns 



55% TRANS. 



44CO 



4600 



4800 



5000 52O0 5400 5600 5800 6000 

 WAVE LENGTH 



Fig. 3. — Energy transmission curve for a 6-inch diameter filter equipped 



with vanes. 



the quality of the atmospheric seeing was immaterial. Unfortunately, 

 the sharp peak of the photoelectric cell sensitivity proved fatal to 

 the success of the observations. Small traces of stray light of bluish 

 color were so disproportionately eiTt'ective as to mask real values in 

 the ultraviolet and the red. Dr. Abbot hopes to develop a sufficiently 

 sensitive black receiver as a substitute for the photoelectric cell in 

 future stellar work. 



Figure 3 shows the energy transmitted by a 6-inch diameter filter 

 (described below and detailed in fig. 4), using the parallel rays of 



