454 Proceedings of Royal Society of Edinhurgh. [july 16; 
would rise more steeply at the beginning than the green one. 
Kunkel’s statement, however, applies only to violet and green rays 
of equal physiological intensity. It is quite certain that in none of 
my experiments was the violet as intense as the green, and the more 
intense any of the colours is, the steeper, of course, will be its curve. 
Besides, the violet curve may reach its maximum before the green, 
and still he flatter at the beginning, since in fig. 4, would 
need to be drawn much farther to the right than it is, if the time 
required for the maximum excitation were to be represented on the 
same scale as the rest of the figure. 
Influence of Change of Intensity of the Light. 
Some interesting observations were made upon this point. If, 
while the greenish phase is being got, the intensity of the light is 
increased, the speed of rotation being kept the same, the appearance 
changes to the violet. If, on the other hand, the intensity is 
reduced, it changes to the reddish. An explanation is found in the 
fact that the curve of excitation for each of the colours rises more 
steeply the stronger the light ; but, since a curve can never become 
perpendicular to the axis, the change must be most appreciable in 
the curves which at the given speed are least steep. 
Take, e.g. (fig. 4), the vertical afhfafbfafhf. Here the violet 
and green ordinates being the smallest, they will be increased pro- 
portionately more than that of the red. In fact, the effect will be 
the same as if this vertical line were moved to the right. If the 
increase of intensity is considerable, but brought about by several 
steps, the whole series of phenomena may be traced in reverse order 
from reddish to violet, without altering the speed. 
This only holds for a frequency of stimulation just about that 
required for fusion, and has no bearing on the discussion at the end 
of the paper. 
Experiments with Coloured Light. 
Here the changes, although susceptible, I believe, of the same 
explanations as in the case of white light, varied, of course, according 
to the colour used, and, therefore, it will be necessary to quote the 
observations in some detail. Bed, blue, green, and yellow (sodium) 
were used. 
