594 ME WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 



the intensity of the impression is precisely the same in both cases. But at the 

 velocity of 40 revolutions in a second, there are twice as many flashes in a given 

 time as there are at the first velocity ; and if the brightness of the flashes was 

 the same as at the first velocity, the brightness of the impression produced by 

 them would be doubled. Since, therefore, the impression, instead of being doubly 

 bright, remains the same as at first, each flash at 40 revolutions in a second must 

 only be half as bright as at 20 revolutions in a second. In like manner, by sup- 

 posing the velocity increased to 80 revolutions in a second, it might be shewn 

 that the brightness of the flashes is again halved. But the efiect of doubling the 

 velocity is to halve the duration of the flashes, therefore the brightness of the flashes 

 is proportional to their duration. This law of vision may be thus stated : When 

 fight of a given intensity acts on the eye for a short space of time, the apparent 

 brightness of the luminous impression on the retina is exactly proportional to the 

 time during which the light continues to act. From the velocities of the discs, 

 and the angles of the sectors used in the experiments, it will be seen that this 

 law is true for impressions lasting from jgL to jL of a second ; and it will 

 presently be shewn to be true for impressions of longer duration. 



V. Observatiotu of the apparent Brightness of Luminous Impressions of short duration. 



In almost all the experiments hitherto described, the phenomena of vision 

 which have been investigated have been derived from the observation of the 

 aggregate effect of luminous impressions succeeding each other so rapidly as to 

 produce a continuous impression on the eye. It is obvious, that such expe- 

 riments aflbrd no information regarding the absolute brightness of the sepa- 

 rate impressions which are thus blended together. I adopted the indirect 

 mode I have now described of ascertaining the connexion between the duration 

 and apparent brightness of luminous impressions, from an apprehension of the 

 difficulty of comparing the brightness of a constant light with that of an isolated 

 flash. But repeated trials satisfied me that my fears were groundless ; and the 

 succeeding experiments prove that, with a little practice, the eye is perfectly capa- 

 ble of making this comparison. Such experiments cannot, however, be long con- 

 tinued without fatiguing the eye, and a considerable effort of attention is re- 

 quired for their successful performance. 



In order to find the intensity of separate impressions of short duration, I 

 used a disc of wood two feet in diameter, revolving once in a second ; so that a 

 sector, whose arc had a known ratio to the circumference of the disc, passed at 

 each revolution before the aperture in one of the screens of the selaometer. In 

 this manner, a series of perfectly isolated impressions was obtained ; and the 

 intensity of each could be compared with that of a light seen by continuous 

 vision in the manner already described. The different sectors were cut in paste- 

 board, and placed over an aperture in the disc. The following experiments were 

 made : — 



