﻿Phenomena in Photometry. 133 



wri<er, it seems impossible that the green illumination could 

 be brighter and yet the field of view appear red. 



Two 100-volt 8 candle-power lamps screened with red and 

 green glass in the usual manner were used for the experi- 

 ments. They were first run at 100 volts, and compared against 

 each other at different distances. The two lamps were then 

 run off 50 volts only, so as to produce a very low illumination, 

 and the experiment repeated. The following table shows 

 very clearly the influence of the Purkinje effect. 



At the higher illumination the readings differed among 

 themselves considerably (as was only to be expected with 

 such a great colour contrast), but they do not seem to be 

 connected in any way with the illumination. 



The readings at the low illlumination can only be regarded 

 as very approximate, but they bring out very clearly the 

 accentuation of the green as the illumination gets weaker. 



Lamps run at 100 volts. 



Lamps run at 50 volts. 



Distance between 

 Lamps. 



Ratio — 



Green. 



Distance between 

 Lamps. 



Ratio Refl 

 Green. 



inches. 

 50 



60 



70 

 120 

 150 



2-32 

 226 

 2-25 

 215 

 2-25 



inches. 

 40 



50 



60 



70 



90 



20 

 1*5 

 11 

 07 

 06 



The conclusion the writer draws from these experiments is 

 that Flicker photometers are affected by the same colour 

 phenomena which affect ordinary photometers. 



The interesting assertion was made by Messrs. Simmance 

 and Abady, in a paper before the Physical Society*, that a 

 colour-blind person obtained practically the same results with 

 their Flicker photometer as people with normal sight. On 

 the other hand, Sir Wm. Abney, speaking of colour-blindness, 

 remarks f : — 



" We cannot hope for instance that the red-blind, who sees 

 no red in the extreme end of the spectrum, would show any 

 luminosity in that region. . . . One of the most striking 

 experiments in colour-vision is to place a bright-red patch 



* Phil. Mag. vii. p. 341 (1904). 



t ' Colour Vision/ p. 83. 



