572 STUDIES IN ADVANCED PHYSIOLOGY. 



and green points persist. In the author's own experience 

 such a disappearance of the blue when falling right in the 

 fovea centralis seems unquestionable from repeated experi- 

 ments. There is, however, a cause that might produce the 

 disappearance of the blue point apart from the absence of 

 the cones. It will be remembered that the fovea centralis 

 lies in the yellow spot, and we know that yellow light very 

 rapidly absorbs blue light. Blue light does not readily pass 

 through a yellow window pane, and so the disappearance of 

 a blue point on the retina may be due to the fact that the 

 yellow pigment of the yellow spot absorbs the blue light and 

 so prevents it from affecting the retina. 



According to the Helmholtz theory, however, the blind- 

 ness of the yellow spot with reference to blue is due to the 

 absence of the rods. The rods being distributed over all 

 other portions of the retina explains why blue is thus gen- 

 erally present. It will also be remembered that the vision 

 purple found in the eye occurs only in the rods, and so this 

 theory goes a point further suggesting that it is a chemical 

 action of the vision purple produced by the light that irri- 

 tates the rods and so occasions a nervous impulse leading 

 to the perception of blue. 



Monochromatic eyes are upon this theory explained as 

 eyes in which the rods alone are functional. The yellow 

 spot is then entirely blind, and to this blindness of the yel- 

 low spot is attributed the difficulty in focussing and the 

 habitual squinting which marks monochromatic persons. 

 The points of acute vision in these monochromatics not be- 

 ing a definite spot, but a circle around that spot, the at- 

 tempt to keep the focus on this circle occasions their extra 

 efforts in looking closely. 



But even normal eyes are monochromatic at times. 

 When the ordinary light of day is very much reduced, as it 

 is some time after sunset, we notice how the sensation red 

 disappears first, then green, while blue is the last sensation 

 to go. Every one must have noticed that a night scene has 

 a bluish cast. Pictures of night scenes are purposely tinted 



