724 Messrs. H. E. Ives and E. F. Kingsbury on the 



compared lights, the higher the speed must be, with consequent 

 loss in sensibility *. 



The most interesting point now appears. Having raised 

 the speed to the value where no alternation of hue is per- 

 ceptible, what is the effect of altering the relative brightness 

 of the two lights under comparison r It is readily seen that 

 if either light, such as A , be reduced to zero, then the points 

 a and ft are of the same hue, namely B . Similarly with 

 the light B . At intermediate values the hue difference will 

 have values between zero and the value at the point of photo- 

 metric balance. In short, when the photometric balance is 

 disturbed, causing " brightness flicker," the two hues which 

 are being alternated become less different and the " colour 

 flicker " actually becomes less. Consequently at the point 

 of colour-fusion and at speeds above it, the only flicker which 

 can be produced is " brightness flicker." 



From the equations previously given it is readily possible 

 to calculate in just what proportions the two compared colours 

 are mixed for various relative intensities and speeds. Thus 

 from equation (2) we have at any time t 



For infinite speed («= oo ) = = ^ . For any finite constant 



n 

 A, 



-B B / Xv/ w r / -i \ 



le~' 2K 2 cos|_^-X^/^ -7T I +l\ 



A_ 



\e -^2 cos 



A_A 

 B B 

 value of co, ^ has a maximum value 



W B 



and a minimum value A 



o 



Bo 



Choosing arbitrary values X = l, K'i = ± K 2 = J, (a) becomes 

 — _ ^ \ l + e ~ V " cos [cot- y/&) i 

 B ~B Ll_^- V^cos K cot- Vw)\' 



* 



Another factor which may he of importance is the different lag of 

 different colours. As pointed out in a previous note, the effect of such 

 a differential lag- would be to prevent the exact dovetailing of the two 

 impressions. This produces a loss of sensibilitv which cannot be cured 

 by a reduction of speed. The lower sensibilitv of the flicker photometer 

 at low intensities has every appearance of being caused by such a failure 

 of the maximum of one colour impression to coincide with the minimum 

 of the other. 



