231 



can thus be compared with great nicety. The light behind the re- 

 volving disc is kept at a constant distance from the screen during an 

 experiment ; and, before causing the disc to revolve, the apertures 

 are made equally bright by varying the distance of the other light 

 from its screen. When the disc is put in motion, the apparent 

 brightness of the aperture behind it is instantly diminished ; and the 

 equality of the apparent brightness of the apertures in the screens is 

 restored, by increasing the distance of the light from the other 

 screen. The ratio of the brightness of the impression produced by 

 the light during the revolution of the disc, to the brightness of its 

 impression, when seen by uninterrupted vision, is that of the squares 

 of the distances of the other light from the aperture in its screen. 

 The following are the principal results obtained by means of this 



apparatus : — 



(1.) When the eye receives, from a light of constant intensity, a 

 succession of flashes of equal duration, which succeed each other so 

 rapidly as to produce a uniform impression, this impression will also 

 have a constant intensity, provided the number of flashes in a given 

 time varies inversely with the duration of each flash. 



(2.) The brightness of the impression produced by flashes of light 

 of a given intensity, which succeed each other so rapidly as to pro- 

 duce a uniform impression on the eye, is proportional to the number 

 of flashes in a given time. 



(3.) When light of a given intensity acts on the eye for a short 

 space of time, the brightness of the luminous impression on the retina 

 is exactly proportional to the time during which the light continues 

 to act. This law has been proved to be true for impressions lasting 

 from j^U^ to 51 of ^ s^'^^"''- ^^® intensity of the impression 

 produced by light which acts on the eye for y^o of ^ second, is 

 almost exactly tV of the brightness of the light when seen by un- 

 interrupted vision; and it is also ascertained that light requires 

 about the tenth part of a second to produce its full effect on the eye. 

 (4.) It is found that lights of diff"erent intensity act on the eye with 

 equal rapidity, so that even the light of the sun produces an impres- 

 sion with no greater rapidity than that of a common gas flame. 



(5.) Rays of different refrangibility act on the eye with equal ra- 

 pidity. 



(6.) Since Professor Whcatstone's experiments have proved that 



