750 PJiotometry of Lights of Different Colours. 



the left by X— AX ; third, both sides by X — AX ; fourth, the 

 right-hand side by X— AX, the left by X. From these two 



values of the ratio 



X-AX 



were obtained and their mean 



was adopted. These values were plotted as a curve, and 

 from this was built up the luminosity curve shown in fig. 2, 



full line- EquAury oraryiGHTHcss ay small steps 



DASHED LIME- - - ' - ~WH/TE * COMPAftlSOtf HELD 



CIRCLES - FLICKER METHOD 



JZ J+ J6 JQ .60 .62. .64- yU 



a curve which may fairly be claimed to be independent of 

 the psychological errors illustrated by fig. 1. 



After obtaining this luminosity curve, a flicker curve and 

 an equality of brightness curve, by the ordinary method, 

 ao-ainst incandescent lamp light, were made. These are 

 shown as well in fig. 2. It will be seen that the flicker 

 curve (circles) agrees with the special small step equality 

 of brightness one. The ordinary equality of brightness 

 curve happens to be quite different from the flicker curve. 



This result is in agreement with the conclusion drawn 

 from the previous work with five observers, that when the 

 psychological errors are eliminated the equality of brightness 

 and the flicker photometers agree at high illuminations. 

 In the one case an attempt was made to eliminate the 

 psychological difficulty by taking the mean of several dif- 

 ferent observers, in the second case by a special method of 

 observation. In the writer's opinion these results justify 

 the opinion that the flicker photometer gives, under the 

 conditions specified, what may be considered the true 

 brightness. 



