﻿Phenomena in Photometry. 123 



In each case a series of readings was obtained with the 

 eye at different distances away from the illuminated surfaces. 

 It should be mentioned that in the case of the Lummer- 

 Brodhun photometer, the telescope was removed while the 

 readings were taken. As will appear later, however, con- 

 siderable variation is possible even with the telescope in 

 position, as in use. 



Curves (1), (2), and (3) speak for themselves. They 

 bring out two points. Firstly, that the ratio of the candle- 

 powers of the red and green lights, as thus observed, is quite 

 different for each photometer. 



Secondly, that this ratio depends on the distance of the 

 eye from the photometer, the red becoming more and more 

 accentuated as the eye recedes. Also the red is most accen- 

 tuated and the curve is steepest for the Lummer-Brodhun 

 photometer, in which the field is smallest. 



Curve (4) was obtained by placing in front of the blocks 

 of the Joly photometer a paper screen which reduced its 

 linear dimensions by one half. The reduction in size of the 

 blocks corresponds with a still further accentuation of the 

 red. 



Fig. 2. — Curves for Joly Photometer. 



1-8 

 1-7 

 1-6 

 1-5 

 14 

 !-3 

 1-2 

 M 

 1-0 



































































K-Cj) 























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; 



r 





































































10 



100 



20 30 4-0 50 60 70 80 90 

 Distance of eye from blocks, in centimetres. 



(1) Glow-lamp compared with Nernst Lamp. 



(2) Metliven Gas Standard compared with incandescent mantle. 



This effect has been found to be quite distinctly observable 



in several commonly occurring comparisons. 



Fig. 2 exhibits 



