Photometry of Lights of Different Colours. 363 



absent under less extreme conditions. It may be much 

 more marked under the doubled illumination which holds 

 when two colours are matched in the flicker photometer. 



Fig. 6. 



Critical Frequency-Illumination Relations for large and small 

 Photometric Fields. 



If the latter supposition is correct it virtually means the 

 adoption of the second explanation, namely that the critical 

 frequency phenomena probably follow the straight line 

 relationship, and that the flicker photometer, influenced by 

 various factors, merely follows this relationship closely but 

 not exactly. In fig. 1, curve A, taken at a higher illumination 

 than those previously mentioned, the critical frequency 

 curve shows a larger shift toward blue than does the 

 flicker, while in curve C the shift toward red is greater. 

 Both these differences are near the limit of accuracy of the 

 measurements, but are consistent in indicating that the 

 flicker photometer method does not follow the critical 

 frequency method absolutely. On the whole it appears 

 more probable that the critical frequency phenomena do 

 obey the simple law given above and that the flicker photo- 

 meter phenomena merely approximate thereto. Other 

 factors, such as speed, mutual action of different colours, 



