i 7 6 



THE HUMAN BODY. 



maximum to its minimum, it is long in proportion to the 

 brilliancy of the light, that is to say, as the disk is white, 

 yellow, red, or blue. 



Several physiologists explain the formation of accidental 

 images by persistent excitation of the retina with diminution 

 of sensibility. They think that the light proper of the retina 

 plays a part in this phenomenon. 



Fig. 41. Irradiation. 



Irradiation, accidental fringes of light. When one portion 

 of the retina is excited by the luminous rays, the vibration 

 is extended to the neighbouring portions, and more strongly 

 in proportion as the light is white ; the result of this is, that 

 of two objects of equal dimensions but of different colour, 

 the lighter one in colour appears the larger in size. If 

 a black circle is traced on a sheet of white paper, and a 

 white circle of the same size on a sheet of black paper, and 

 both placed at an equal distance from the eye, the white one 

 appears larger than the black. In the same way, if we make 

 a disk half white and half black, the white half appears the 

 larger. In both cases the white encroaches upon the black, 

 because the impression made by it upon the retina is more 

 vivid, and the longer the experiment is continued the greater 

 appears the difference in diameter. The name of irradiation 

 has been given to the group of phenomena of this nature. 

 It is the same cause which produces a ring of complementary 

 colour around an image impressed on the retina by a coloured 

 object. If a square of red be placed upon a white ground, 



