384 PHYSIOLOGY CHAP. 



the different parts of the retina, the second the differences which 

 they present in regard to colour. 



The different forms of total and partial colour-blindness, which 

 resist interpretation on Bering's theory, can be rationally explained 

 by Schenck's hypothesis as follows : 



(a) The deuteranopia of v. Kries (erroneously known as " green- 

 blindness") is a reduction-system from normal colour- vision. It 

 agrees thus far with the red-green blindness of the middle retinal 

 zone, that affected people describe only white, yellow, and blue 

 sensations, and have no sensation of red, which is perceived as 

 yellow, or of green, which seems colourless. The deuteranopic 

 differs from the normal eye in that the red-green blindness is not 

 confined to the middle zone, but extends to the central part of the 

 retina, and does not disappear on increasing the area, intensity, 

 and saturation of the object. 



On Schenck's theory deuteranopia is explained quite simply by 

 assuming that owing to arrested development in the cones the 

 second phase in the cleavage of the yellow substance into the red 

 and the green substance has not taken place. 



(6) The protanopia of v. Kries (erroneously termed " red-blind- 

 ness") is a reduction-system, which differs from deuteranopia by 

 a different distribution of spectral luminosity, but is practically 

 identical with it in all other respects. Protanopes, according to 

 v. Kries, show diminished sensibility to light of long wave-length, 

 so that the more highly refractive part of the spectrum is shortened, 

 the red appears very dark, and the yellow less bright than in 

 normal individuals. The sensibility to light of short wave-length 

 1 from about 500 to 391 /z/x, or from the end of the .yellow to 

 the end of the violet appears, on the contrary, to be relatively 

 greater in protanopes than in deuteranopes and normals. So that 

 while red seems much darker to protanopes, green and blue are 

 rather brighter to them than to normal people. 



To explain protanopia on Schenck's theory it must be assumed 

 that the receptor or resonator for rays of long wave-length is 

 absent, while the stimulator of red sensations is present ; and that 

 the second phase in the cleavage of the primitive visual substance 

 has not taken place. 



(c) Blue -yellow llindness is a rare form of congenital dys- 

 chromatopsia. Up to the present four cases only have been fully 

 investigated. Individuals who are blind to blue and yellow 

 describe only three light-sensations white, bluish-red, and bluish- 

 green. They call the lights which normals see as white, yellow, 

 and indigo-blue white ; the light of long wave-length red ; that of 

 medium wave-length green. Like normal individuals they recognise 

 the uniform luminosity of different homogeneous or mixed lights, 

 as well as the varying brightness in the different parts of the 

 spectrum. We may therefore conclude that in this form of partial 



