384 VISION. LECT. XX. 



at any luminous body, close the eyes and you will still 

 see it. 



In order to ascertain the length of this duration, Aime 

 contrived to revolve, in opposite directions, two discs 

 fixed on one axis. One of them was perforated with a 

 great number of holes, in the form of equal and symmetri- 

 cally disposed sectors; the other had but one such hole. 

 When a fasciculus of light was made to fall on this appa- 

 ratus while in motion, in a dark place, the eye which looks 

 along the common axis of the two discs, perceive some- 

 times one illuminated sector, whose position is variable and 

 dependent on the coincidence of the single aperture of the 

 second disc with each of those of the first ; sometimes two, 

 sometimes three, or even more, and, lastly, one disc of light. 



These different impressions depend on the rapidity of 

 rotation. There is only one sector if the velocity of motion 

 be such, that the second coincidence takes place when the 

 impression made upon the retina by the first has ceased. 

 There are two sectors if the impression continues until the 

 second coincidence takes place, and so on. It thus becomes 

 easy by this apparatus, to determine the duration of the 

 impressions upon the retina. Some very ingenious appa- 

 ratus, which likewise serves to amuse children, have been 

 constructed on the same principle as that of Aime's instru- 

 ment just described. Around a circle is placed a number 

 of figures of men, exactly alike in dress and shape, and all 

 represented in motion. They are arranged successively, 

 one after the other, each representing a different but suc- 

 cessive position of a given exercise, as, for example, 

 sawing, playing the violin, dancing, &c. The first circle 

 is seen through the slits of a second one. By turning both 

 upo.n one axis, the eye receives the impression of each of 

 the positions of the figure, at the moment of the passage of 

 the corresponding opening, and preserves the impression 



