358 O. N. Rood on Revolving Dises. 
bination takes place; here, while one eye has objective white 
light presented to it, the other retina is affected by a rapidly 
fading subjective impression, so that the two impressions are 
during most of the time of unequal intensity ; the result is lustre. 
lutions per second there is a loss of definition, and directly the 
appearance becomes a little puzzling; with a higher rate, as for 
example 4,2, per sec., the disc takes on a very remarkable appear 
ance, described by some as flickering, by others as “glitering. 
To make a little examination of it undisturbed by its surround: 
ings, I cut a circular aperture 2 inches in diameter in a large piece 
of card-board, and viewed through this with a single eye a por 
tion of the revolving disc. The appearance presented I can de- 
scribe in no other terms than by calling it lustrous, with rapid 
variations in the intensity of the light. In this case the stro 
objective light is seen through the weaker fading subjective 1” 
ression, and the latter is of course at regular intervals percel¥ 
distinct by itself, so that the eye is in effect acted on by two 
masses of light of unequal intensity, and is also sensible of theit 
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