THE EYE AND THE PHYSIOLOGY OF VISION. 573 



blue. A moonlight scene in a picture is readily distin- 

 guished from a view by day in the absence of all reds, and 

 the prevailing tint of blue. In subdued light red and green 

 disappear, and things before they shade into blackness 

 seem to pass through gradations of deeper and deeper blue. 

 According to this theory we have lost the use of the cones 

 and our vision is limited to the rods. Therefore, we, too, 

 ought to be blind in the yellow spot, as this possesses no 

 rods, and the holders of this theory in support of this, point 

 out the difficulty we have in distinct vision even when the 

 light would still be strong enough to permit the details to 

 be seen carefully. 



Dichromatic eyes are explained by supposing that in ad- 

 dition to the rods which see blue, there are cones which 

 see yellow, and the two varieties of dichromatic eyes are 

 explained by the relative ease with which these yellow 

 cones are stimulated. In red-blind individuals they are 

 stimulated most readily by green objects; in green-blind 

 persons the yellow cones are stimulated most readily by 

 red objects. 



NORMAL OR TRICHROMATIC EYES. 



In the normal eye this theory supposes, as was stated, 

 three sensations: blue, green and red. A grievous fault of 

 this theory is its inability to explain why the second color 

 in dichromatic eyes should be yellow, and why this color 

 should not be represented at all in the trichromatic eyes. 

 One would naturally suspect that in normal eyes the ad- 

 dition would be made to the blue and yellow of partially 

 color-blind eyes. It was, in fact, this difficulty to explain 

 how the yellow of dichromatic eyes was replaced by green 

 and red of the trichromatic eyes that led to the proposal of 

 the Hering theory, which explains this point at least satis- 

 factorily. 



It will be noticed that a normal eye might be derived 

 from the composition of a green-blind eye and a red-blind 

 eye. Thus dichromatics see blue just as we do. This is 



