O. N. Rood on Revolving Discs. 357 
Arr. XXXVI.—On certain Appearances produced by Revolving 
Dises ; by Prof. O. N. Roop. 
- Dove, some years ago, succeeded in producing a lustrous ap- 
pearance, by the binocular combination of geometrical figures, 
A circular disc of white card-board, 9 inches in diameter, with 
f its surface painted of a dead black, was caused to rotate by 
clock work at varying rates, while the bright light from a win- 
Night-hand aperture some of the white portion of the disc was 
It was found that with slow rates of rotation (2,3, revolutions 
Per sec.) the strength of the lustre was not impaired, and it was 
Just as plainly perceptible with more rapid rates. 
But when the disc was made to revolve so fast that its surface 
Seemed covered by a uniform tint of grey, and the so-called 
flickering had ceased, no lustre in the proper sense of the term — 
coul seen, the appearance being exactly that which is pre- 
_ Sented to a single eye under similar circumstances. 
When a dise of this kind revolves at such a rate as to we 
of a uniform tint, the duration of the impression produced on 
eye Wy the white half lasts with undiminished force while 
the black half is passing before the same eye, so that while the 
. nght eye is being objectively impressed by the white surface, 
the left eye has retained a niljecuhe impression of exactly the 
_ Same nature and strength; both eyes are then really in effect im- 
_ Pressed all the time in exactly the same way, and in consequence 
_ Of this no lustre is perceptible. But when the rates of rotation 
_ 4re lower than that above indicated, a different binocular com- 
_ * Farbenlebre, pp. 171 and 177. * This Journal, May, 1861, _ 
_ Aaé Jour. Scr.—Secoxp Serius, Vor. XXXV, No. 105.—Mar, 1863. ue 
