088 MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 
ments with discs having sectors varying from 60° to 7°30’, and did not appear to 
differ very sensibly in different cases ; but it seems probable that the velocity re- 
quired to produce a continuous or a uniform impression should be sensibly af- 
fected by greatly altering the angle of the sector. 
Ill, Effect of combined Luminous Impressions on the Eye. 
So long as the rotation of the disc is so slow as to allow each flash to be seen 
separately, the brightness of the flashes diminishes as the velocity of rotation 
increases, until, at about twenty revolutions in a second, the flashes become 
blended into a nearly uniform impression. Whenever this takes place, no 
farther increase of the velocity of the disc diminishes the intensity of the impres- 
sion in the smallest perceptible degree. This result is evidently produced by the 
increased number of luminous impressions in a given time compensating for their 
diminished intensity; but it is remarkable that the one effect should so exactly 
compensate for the other. Having found that this compensation took place at 
velocities varying from twenty to forty revolutions in a second, I was anxious to 
ascertain whether it continued unimpaired at higher velocities. For this purpose 
a disc of pasteboard 4:5 inches in diameter, with a sector of 2° 30’ cut out of its 
margin, was fitted to the axle of a clockmaker’s wheel-cutting engine. It was 
found by a previous careful trial, that the disc made exactly 100 revolutions for 
each revolution of the driving-wheel; and as the latter, at its greatest velocity, 
made thirteen revolutions in ten seconds, the disc ought to have revolved 130 times 
inasecond. But to avoid the chance of errors arising from the driving-bands slip- 
ping at so high a velocity, I availed myself of Professor WHEATSTONE’s Ingenious 
method of ascertaining the velocity of a rapidly-revolving axle, described in his 
paper in the Philosophical Transactions for 1834, to which I have already re- 
ferred. This consisted in observing the pitch of the note produced by the rapid 
percussion of a pin, fixed in the revolving axle, upon a piece of paper held in con- 
tact with it. The highest note produced during the experiment was rather less 
than an octave below C of the tenor clef, which corresponds to above 128 vibra- 
tions ina second. This result agrees almost exactly with the calculation founded 
on the observed rotation of the disc at low velocities; and it may, therefore, be 
concluded, that the disc made above 128 revolutions in a second. Since the are 
of the sector was ;4q of the circumference, the light from a luminous point placed 
behind the disc would, at each revolution, act on the eye for only 7433 of a second. 
A lighted candle being placed behind the disc, the machine was put in motion, and 
the velocity gradually increased until the driving-wheel made thirteen revolutions 
in ten seconds, after which it was allowed to come to rest spontaneously. It was 
found that the brightness of each successive flash diminished as the velocity of 
the disc increased, until the impression on the eye became uniform, at a velocity of 

