446 
Proceedings of Royal Society of Edinburgh, [july 16 , 
gentlemen to turn the mirror, and, without knowing what they were 
to look for, to describe what they saw. There could he no doubt 
about the general agreement. The shade of a colour was sometimes 
differently named; hut, as to the order in which the colours followed 
one another, there was no difference of opinion.* 
Gas-light, sun-light, candle-light, and the light of a petroleum 
lamp have all been used ; and although, as might be expected from 
the varying proportion of the different spectral colours in those 
different lights, one or other of the stages to he described below 
may have been more or less strongly marked, I have never been 
unable to trace them all in every case, except when the light was 
of more than moderate intensity. 
A description of a typical experiment will best explain what 
was observed. S means “ speed of rotation,” + S means “ increased 
speed.” 
Experiment 1. — Gas-light; dark room; distance of light from 
mirror, 2 feet ; distance of eye froni mirror, 1 foot. Begin with 
slow rotation, and increase. 
Slow S : A faint green band appears at each side of the broad 
yellowish-white band, which represents the successive 
images of the gas flame. 
-f S : The green band becomes dark green and very sharp. 
Inside it is a distinct broad violet band. 
-f S : The green band gets darker, and the rest of the image 
inside it becomes dashed with what look like 
yellowish-green ripples. 
-I- S : Green band at edges disappears. The yellowish-green 
ripples become fused, and the whole image is now 
distinctly greenish. 
+ S : Whole image becomes reddish-brown. 
-f S : bTo further change. 
* The observations need some little practice, and hardly anybody has been 
able, at the first trial, to see all that is to be seen. When the rotating disk 
is used, it is a good plan to have an arrangement by which the length of the 
slit may be increased or diminished. Keeping now the distance of the source 
of light fixed, we can run through the phases either by alteifing the rate of 
rotation or the length of the slit. Keeping the rate of rotation and length of 
slit constant, we can do the same by altering the distance of the light. The 
length of slit should not be large in comparison with the circumference of the 
disk. 
