OF LUMINOUS IMPRESSIONS ON THE EYE. 35 
The nature of this arrangement will be understood from fig. 1, Plate L., 
in which A, B, ©, a, 6, ¢ represent a train of toothed wheels and pinions 
driven by a weight suspended from a line, which is coiled round a drum on 
the axis of the wheel A, and by which motion is communicated to the disc D. 
The wheels B, C have each 120 teeth, and the pinions 0, c, 12 teeth; so that if 
B revolve once in a second, C revolves 10 times, and D, 100 times in a second. 
Apertures I, K are cut in the wheels, which are otherwise solid, and the whole 
is so arranged, that the apertures I, K, the sector S, and the aperture T in a 
screen H illuminated by a light F, are in the same straight line; so that an 
observer at E can see the illuminated aperture I through the holes in the wheels 
and the sector in the disc. Since the wheels B, C, and disc D revolve respec- 
tively once, ten times, and a hundred times in a second, if the apertures are in 
line at. the commencement of any one second, they will again come into line at 
the commencement of each successive second. Suppose the wheel B to be 
removed, the wheel C and disc D continuing to revolve. By the time the disc D 
has made a single revolution, the aperture K has moved round so far that when 
the sector S has returned to its original position, the wheel C intercepts the light ; 
and it is obvious that K will not return to its proper position for transmit- 
ting light to the eye until the wheel C has made a complete revolution. The 
effect of the combination of the wheel C and disc D will therefore be to trans- 
mit to the eye a single impression at every revolution of C, or at the end 
of every ;,th part of a second. Supposing, as before, that the sector D is 
~ith of the circumference, the eye will receive only 10 impressions in a 
second, each of which has been made in ;,,;,,th part of a second. We may 
now dismiss the disc D from our consideration, and regard the wheel C as a disc 
revolving 10 times per second, and having an excessively narrow sector cut in it, 
occupying ;,2,,th of its circumference. By repeating precisely the same reason- 
ing for the wheels B and C as has been employed for the wheel C and disc D, 
it will be seen that the wheel B will transmit to the eye a single impression at 
each revolution, or once a second, derived from the wheel C. The wheel B will 
therefore transmit to the eye a single impression in a second, which has acted on 
the eye for —,;th of a second, and will thus be equivalent to a disc revolving 
once a second, and having a sector of only =2,,,th part of its circumference. It 
will be seen also, that as the smallest practicable angle for the sector is obviously 
the same, whatever be the size of the disc, the effect of the arrangement will be 
to make a disc of a foot in diameter equivalent in efficiency for observing short 
impressions to one of 100 feet in diameter. 
It may, at first sight, be supposed that only a single screen-wheel, as it may 
be called, such as C or B, would be required. Such, however, is not the case. 
The wheel C alone would be insufficient; for when the disc D was driven with 
high velocities, C would revolve so fast that its transmitted impressions would 



