444 Proceedings of Royal Society of Edinburgh, [jult 16, 
the spectroscope, as it was weakened too mucli when it was reflected 
from the fixed mirror, 
I do not propose to say anything about the spectroscopic work 
here, as the results are still incomplete. So far as they go they con- 
firm Kunkel’s observations on stimulation with homogeneous rays. 
To return to fig. 1 ; it is easy to calculate the duration of the 
stimulus with a given velocity of the rotating mirror, and a given 
breadth of the reflected image at the eye. Let n be the number of 
revolutions per second, I the distance or (the two distances 
M'ere practically equal), and h the breadth of the image at the eye 
expressed in millimetres. Then we may consider the point where 
the reflected ray, or one edge of it, cuts the fixed mirror, as moving 
in a circle of radius I ; and E the point where the ray cuts the 
retina or the plane of the pupil, as moving in a circle whose radius 
is 2?. Now for each semi-revolution of the mirror the reflected ray 
will make a complete revolution, E will move with a velocity 
of 2. 27 t. 2Z. n per second. 
Say v = ^Trln, where v is the velocity of E. 
The maximum speed of rotation which it was found j)ossible to 
attain was 170 turns per second. Substituting this value for and 
for I its maximum value 10 metres, or in millimetres 10,000, we get 
v = 8 X 3T4 X 10,000 x 170 = 42,704,000 mm. per sec. 
The time during which stimulation lasts is evidently the time 
which the whole ])readth of the beam takes to pass over the 
pupil. Call it Z. We thus have t 
The smallest value of h in 
5 
the experiments was 5 mm. This would give Z = ^ 
704,000 
= ^ 54 Q or in round numbers g 5 qq qqq second was the 
minimum length of stimulation time. This would correspond for 
light in the region of the line B to something like 53,000 vibrations; 
and for line H about 93,000 vibrations. 
It will be seen that by this method the length of a stimulus can 
be conveniently reduced to an almost inconceivable amount. By 
increasing the speed of the rotation, which might be done with a 
specially constructed apparatus; by increasing the distance Z, which 
