1888 .] Mr G. N. Stewart on Intermittent Light. 
447 
Experimeiit 2. — Mirror driven by gas-engine ; petroleum lamp 
for source of light. 
S (1) : Dark green edges; middle greenish-yellow, occasionally 
dappled with pale violet. 
Often can distinguish three zones ; green outside ; in- 
side this yellow or yellowish-green ; inside this a pink 
or violet band, and the central part very pale violet. 
S (2) : Dark green edges have disappeared. The yellowish- 
green zone which was inside them with S (1) has 
increased in breadth, and the central part is mottled 
with greenish-yellow. 
S (3) : Faint, but broad yellowish-brown band at the edges, and 
yellowish-brown mottling over the rest of the image. 
Intensity of Light Increased — 
S (3) The appearance is now the same as was seen, before 
increasing the light, with S (2). 
S (1) = 7 turns a second of the rotating mirror. 
S (2) = 10 „ 
S (3) = 17 „ 
Here the highest speed used was not sufficient to produce the 
final stage. From other experiments, I know that it would have 
needed about 25 turns a second to give it. 
The results of many such experiments may be formulated 
thus For any given intensity of the light there is a rate of revolu- 
tion of the mirror with which the violet preponderates ; with a higher 
speed the green preponderates — luith a still higher speed, the red. 
This is so far merely a convenient summing up of the appearances 
observed. Whether it will also serve to sum up the explanation of 
the appearances we must consider later on. It is necessary to regard 
a particular part of the image, if one would trace the cycle, because 
the intensity of illumination at the edges is not the same as in the 
middle; and, as Experiment 2 shows, an increase of intensity puts 
the phenomenon back to an earlier stage, and corresponds to a 
decrease of speed; while a decrease of intensity puts it forward, and 
corresponds to an increase of speed. 
Since contrast phenomena may mix themselves up with the 
appearances at the edges, I wish first to take the consideration of 
VOL. XV. 30/11/88 2 F 
