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Proceedings of Royal Society of Edinburgh. [jult 16 , 
happens for particular velocities. If after equality has been reached 
with one of those velocities, the speed be increased, the relative bright- 
ness of the stars changes, one decreasing and finally disappearing, 
the other reaching a maximum intensity, to decrease it in its turn. 
Young and Forbes noticed, with a white light source, that 
“ always the light which is increasing with respect to the other with 
increase of velocity of the toothed wheel appears red, and the other 
blue. At each successive equality (e.g., the 11th and 12th, the 12th 
and 13th, &c.) the colours of A and B (the stars) are reversed.” 
Their explanation of the colours was that the velocity of blue light 
is greater than that of red. This would explain the appearances, but 
there are great difficulties in the way of accepting the explanation. 
The physiological conditions of the experiment, it seemed to me, 
were deserving of some attention. We have here an intermittent 
stimulation of the retina. The number of stimuli per second can 
be varied, and so can the length of each. But, while in our 
observations the number of stimuli and the stimulation time are 
inversely proportional, in those of Young and Forbes a small increase 
of velocity of the toothed wheel may extinguish one of the stars, 
i.e., reduce its stimulation time to zero. 
They generally used the 12th and 13th equalities, which corre- 
sponded to a speed of about 410 and 450 revolutions of the toothed 
wheel per second. A difference of speed of about 10 per cent, 
could produce an infinite difference in the brightness, and therefore 
in the time of stimulation. Accordingly, the change in the velocity, 
so far as it affects the number of stimuli in a given time, may 
probably^be neglected. We may consider, in fact, that the number of 
stimuli per second remains constant, while the length of each stimu- 
lation is continuously varied from zero to a certain finite maximum 
value, and from this value back again to zero. My observations 
showed no difference in colour when the speed was increased beyond 
a limit which lay higher the greater the intenisty of the light, but 
which certainly corresponded to a frequency of stimulation far less 
than that of Young and Forbes’s 12th equality. In any case, mere 
increase of speed could not explain the phenomena, because the 
colours were seen to be reversed with every successive equality. 
Let us consider, therefore, the effect of continuously varying the 
length of time of the stimulus, ^.e., the brightness of the star, and 
