Polarization Flicker Photometer. 377 



9. Effect of Choice of Speed on Flicker Photometer Settings. 



In the first theoretical paper the condition which must be 

 satisfied when a setting is made on the flicker photometer 

 was studied and found to be expressible by the equation 



X X 



2i 1 /v2k: v " = 2i 2 /va:^. m _ (12) 



The assumption was then made (assumption 3 of that paper), 



that the position of setting is independent of the speed, and 



the relationship of the flicker setting to the equal intensity 



and equal critical speed setting was developed for an assumed 



constant speed. In the light of the more recent work this 



assumption is no longer necessary, nor is it rigidly true. 



Inspection of fig. 7, for instance, shows that the bisector of 



the no-flicker region in the case of two different colours is 



inclined toward the colour of lower critical speed, so that 



the higher the working speed the more this will be 



favoured. 



Introducing the complete expression for the diffusivity 



in terms of the critical speed-illumination relation, (12) 



becomes 



-2 — 2 



V«log£- Va>log-s- 



log Ij ° B = log I 2 -= E , (13) 



Vai log I x + />i V«2logI 2 4 6 2 



'2 



in which the values of I x and I 2 which satisfy the equation 

 are the ones which will be adjudged equal by the flicker 

 photometer criterion, when the alterations occur at the 

 speed a). 



This expression shows that the point of setting is different 

 for different speeds, and at the same time that the approach 

 to equal brightness setting is closer the lower the speed. This 

 emphasizes again the importance of avoiding all mechanical 

 flicker and of selecting the speed for each colour difference. 



Obviously co should be the critical speed for the appear- 

 ance of colour flicker, which has been called g>m above. 

 This is given in terms of 8r and S B in equation (11), which 

 transposed is, 



/log 

 w m = « — «- (14) 



Substituting this in (13), the complete expression for the 

 behaviour of the equal exposure flicker photometer, when 

 Phil. Mag. S. 6. Yol. 33. No. 196. April 1917. 2 C 



