Theory of the Flicker Photometer. 



309 



measurements on a totally colour-blind observer (Uhthoff), 

 and found that the critical speed for him was far below that 

 for a normal observer and seemed little affected by varying 

 the illumination. Unfortunately their account is very 

 meagre of exact data, and they appear to have discontinued 

 the experiments after merely establishing that there is a 

 difference between the two classes of observers. 



5. The Flicker Photometer with Unequal Exposures 

 of the Compared Colours. 



In the design of the flicker photometers a question apt to 

 require answer is, What effect, if any, is to be expected from 

 an unequal exposure of the two fields? An illustrative case 

 is furnished by those flicker photometers which alternate 

 the two fields by the oscillation of a lens or mirror. It* the 

 amplitude of the oscillation is small, one side of the field is 

 exposed for a majority of the time to one colour, the other 

 side for a majority of the time to the other colour. Again. 

 if the mechanical centre of the alternating system does not 

 coincide with the optical dividing line unequal exposures 

 result. What will be the effect on the readings ? 



A qualitative answer to this question is furnished on the 

 basis of our fundamental assumptions by consideration of 

 fig. 9. Here are represented two complementary dovetailed 



Fie. 9. 



Diagrammatic representation of behaviour of Flicker Photometer with 

 unequal exposures of the compared colours. A and B represent the 

 transmitted stimuli as given by two complementary openings. These 

 have approximately the same fractional range. Their dovetailed sum- 

 mation S x shows outstanding nicker. On increasing B to B', the 

 amplitude of fluctuation is increased not only in the proportion of B' to 

 B, but still more due to the increased diffusivity. The new summation 

 S is constant. 



