﻿128 



Mr. J. S. Dow on. Colour 



becomes noticeable, in practice the following experiment 

 was carried out : — 



Two 100-volt 8 C.P. glow-lamps were run in series w r ith a 

 constant P.D. of 190 volts across them. 



One was screened with red glass, and the other with green 

 glass. The distance between the two lamps was varied from 

 20 to 250 inches, and the mean of a set of readings, giving 

 their relative candle-power, taken in each case. In order to 

 avoid the effect mentioned in (2) above, the Lummer-Brodhun 

 photometer was used, and the telescope was kept in exactly 

 the same position throughout the experiment. 



Fig. 1 shows the result of plotting the ratio of the candle- 

 power of the two lamps against the illumination of the photo- 

 meter-disk, in candle-metres. 



Fig. 4. 



Fig. 4. — Curve exhibiting- Purkinje Phenomena for Ruby Red and 

 Signal-Green Lights. 



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Intensity of Illumination of Photometer-screen in Candle-metres. 



From what has been said above, it is clear that we cannot, 

 strictly, define the candle-power of a red light in terms of 

 white-light standard, because this ratio depends upon the dis- 

 tance of the eye from illuminated surfaces. 



