THE EYE. 



duration of this impression on the retina, after the removal of the 

 visible object which produced it, varies according to the degree of 

 illumination and the colour of the object. The more intense tile- 

 illumination, and the brighter the colour, the longer will be the 

 interval during which the retina will retain their effects. 



51. To illustrate this experimentally, let a circular disk formed 

 of blackened card or tin, of twelve or fourteen inches in diameter, 

 be pierced with eight holes round its circumference, at equal dis- 

 tances, each hole being about half an inch in diameter, as represented 

 in fig. 6. 



Let this disk be attached upon a pivot or pin at its centre o to a 

 board A B c D, which is blackened everywhere, except iipon a 

 circular spot at v, corresponding in magnitude to the holes mac.fr 

 in the circular plate. 



Let this spot be first supposed to be white. Let the circukr 

 disk be made to revolve upon the point o, so as to bring the 

 circular holes successively before the white spot at v. The retina 

 will thus be impressed at intervals with the image of this circular 

 white spot. In the intervals between the transits of the holes 

 over it, the entire board will appear black, and the retina will 

 receive no impression. If the disk be made to revolve with a 

 very slow motion, the eye will see the white spot at intervals, 

 but if the velocity of rotation be- 

 gradually increased, it will be found 

 that the eye will perceive the white 

 spot permanently represented at v, as 

 if the disk had been placed with one 

 of its holes opposite to it without 

 moving. It is evident, therefore, that 

 in this case the impression produced 

 upon the retina, when any hole is 

 opposite the white spot, remains until 

 the succeeding hole comes opposite to 

 ** C it, and thus a continued perception of 



the white spot is produced. 



If the white spot be illuminated in various degrees, or if it bs 

 differently coloured, the velocity of the disk necessary to produce 

 a continuous perception of it will differ. The brighter the colour 

 and the stronger the illumination, the less will be the velocity of 

 rotation of the disk which is necessary to produce a continuous 

 perception of the spot. 



These effects show that the stronger the illumination and the 

 brighter the colour, the longer is the interval during which the 

 impression is retained by the retina. 



52. This continuance of the impression of external objects on the 

 70 



Fig. 6. 



