Chap, hi J 



SIGHT. 



927 



the retina thus illuminated, will, as stated above, follow the 

 same path as on entering, and so return to the focus a. Hence 

 the rays reflected from a number of points on the retina, such 

 as those forming the arrow at a\ will be brought to a focus in a 



Fig. 154. Diagram to illustrate the Principles of a simple Form of 



Ophthalmoscope. 



corresponding number of points at a, i.e. will form an (inverted) 

 image of the arrow at a. And the observing eye placed at A 

 behind the hole in the mirror will see at a an inverted image 

 of the illuminated retina. 



§ 578. As to the meaning of the difference between rods 

 and cones no satisfactory statement can be made. It has, it 

 is true, been suggested that the cones subserve the vision of 

 colour and the rods that of form only. This, however, is in 

 flagrant contradiction to both the theories of colour vision dis- 

 cussed above. For colourless vision of form is the appreciation 

 of differences in black and white ; and according to the Young- 

 Helmholtz theory, white is simply a combination of colour sen- 

 sations. Sensations of white, apart from colours ordinarily so 

 called, are only admitted by Hering's theory, and an extension 

 of this theory in the direction that the rods are connected 

 exclusively with the white and black substance, and the cones 

 exclusively with the red-green and yellow-blue substances, lands 

 us at once in absurdity. Moreover since it is in the fovea 

 centralis that we have the most acute vision of both form and 

 colour, the cones alone must be able to serve as the instruments 

 of all visual sensations. The argument that in nocturnal ani- 

 mals the rods are developed almost to the exclusion of cones, 

 because such animals do not need colour sensations, is one 

 which can be turned against itself, since it may be urged that 



