Mr. A. S. Davis on Recurrent Vision, 529 



against a dark background may be seen. The effect is much 

 better seen by twilight or gasHght than in full daylight. The 

 phenomenon^ however^ is by no means so well observed by this 

 method as by means of a board and shutter,, owing probably 

 to the illumination of the retina not being sufficiently instan- 

 taneous. 



Professor Young, in explanation of the phenomenon noticed 

 by him, suggests the idea that a nerve-current from the eye to 

 the brain may, on reaching the brain, suffer partial reflection 

 back to the eye, and thence again to the brain, and thus give 

 rise to a second sensation. It being now, however, ascer- 

 tained that the colour of the recurrent image is entirely different 

 from the colour of the light which produces it, this explanation 

 appears no longer tenable. 



If we admit the truth of Dr. Thomas Young^s theory of colour- 

 sensation (namely, that the sensation of light is produced by the 

 excitation of three different kinds of nerves, — an excitation pro- 

 duced in one kind giving rise to the sensation of blue light, in 

 another to that of green light, and in a third to that of red hght), 

 the above experiments appear to lead to the conclusion that 

 when any one of the three kinds of nerves is excited at any 

 part of the retina, an excitation is induced in those nerves of the 

 other kinds which have their extremities in the same part of the 

 retina. A slight difference between the recurrent and the com- 

 plementary colour might arise from the mutual action between 

 two kinds of nerves, differing in intensity for different kinds of 

 nerves. 



Thus the fact that the recurrent colour given by blue light is 

 rather greener than the complementary colour of the same light, 

 may arise from an excitation in the blue-light nerves inducing a 

 stronger excitation in the green-light nerves than in the red-light 

 nerves. The great difference in the case of orange-red light be- 

 tween the recurrent colour, which is red-blue, and the comple- 

 mentary colour, which is sea-green, is, I believe^ explained thus. It 

 was noticed that pure red produces no recurrent image. Hence, 

 when orange-red light, which consists chiefly of red and partly 

 of green light, is used, only the green component gives rise 

 to a recurrent image. Hence the colour of the recurrent image 

 should be the same as that obtained with the green glass. It is, 

 however, rather redder; and this, I think, arises thus: when 

 a red glass is used the light must be intense in order that any 

 recurrent image may be seen ; and when the light is intense it 

 continues for a short time after the shutter is closed, giving 

 rise to what is known as a persistent image. This persistent 

 image is superimposed upon the recurrent image and reddens it. 

 With any other colour, the persistent image, if there is any, is 



Phil. Mag, S. 4. No. 396. Suppl. Vol. 41. 2 M 



