CONTINUANCE OF THE IMAGE. 



Let the disk thus prepared be attached to the back of a blackened 

 screen, so as to be capable of revolving behind it, and let a hole 

 one inch in diameter be 

 made in the screen at any 

 point, behind which the 

 zone A B c D is placed. If 

 the disk be now made to 

 revolve behind the screen, 

 the hole will appear as a 

 circular white spot so long 

 as the white space A at 

 passes behind it, and will 

 disappear, leaving the same 

 black colour as the screen 

 during the remainder of 

 the revolution of the disk. 

 The hole will therefore be 

 seen as a white circular spot 

 upon the black screen during one-twentieth of each revolution 

 of the disk. If the disk be now put in motion at a slow rate, 

 the white hole will be seen on the screen during one-twentieth of 

 each revolution. If the velocity of rotation imparted to the disk 

 be gradually increased, the white spot will ultimately disappear, 

 and the screen appear of a uniform black colour, although it be 

 certain that during the twentieth part of each revolution, what- 

 ever be the rate of rotation, a picture of the white spot is formed 

 on the retina. 



49. The length of time necessary in this case for the action of 

 light upon the retina to produce sensation may be determined by 

 ascertaining the most rapid motion of the disk which is capable 

 of producing a distinct perception of the white spot. t This 

 interval will be found to vary with the degree of illumination. 

 If the spot be strongly illuminated, a less interval will be suffi- 

 cient to produce a perception of it ; if it be more feebly illuminated, 

 a longer interval will be required. The experiment may be made 

 by varying the colour of the space A M of the zone, and it will be 

 found that the interval necessary to produce sensation will vary 

 with the colour as well as with the degree of illumination. 



50. Numerous observations on the most familiar effects of 

 vision, and various experiments expressly contrived for the pur- 

 pose, show that the retina, when once impressed with the picture 

 of an object placed before the eye, retains this impression, some- 

 times with its full intensity and sometimes more faintly, just as 

 the ear retains for a time the sensation of a sound after the cause 

 which has put the tympanum in vibration has ceased to act. The 



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