588 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 impi-ession should be sensibly af- 

 fected by greatly altering the angle of the sector. 



III. 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 thuieen revolutions in ten seconds, the disc ought to have revolved 130 times 

 in a second. 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 1 28 vibra- 

 tions in a 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 aljove 128 revolutions in a second. Since the arc 

 of the sector was ^54 of the circumference, the light from a luminous point placed 

 behind the disc would, at each revolution, act on the eye for only jg|^ 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 imiform, at a velocity of 



