CHAP, in.] SIGHT. 121 



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

 ima^e of the arrow at a. And the observing eye placed at A 



M 



FIG. 150. DIAGRAM TO ILLUSTRATE THE PRINCIPLES OF A SIMPLE FORM 

 OF OPHTHALMOSCOPE. 



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

 of the illuminated retina. 



776. 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 discussed 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 sensations. Sensations of 

 white, apart from colours ordinarily so called, are only possible on 

 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 animals 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 

 the dim light in which these creatures move calls for increased 

 and not diminished appreciation of small differences of colour. 

 The coloured globules intercalated between the outer and inner 

 limbs of cones in some of the lower animals, such as birds and 



