Notices respecting Xeic Books. 167 



to us conclusively against any three-colour theory. The cases of 

 colour-blindness which he records provide what will be to many 

 the most convincing refutation of the theory so long accepted. 

 The shortening of the red — or violet — end of the spectrum 

 without defective hue perception ; the great variety of defects of 

 colour-vision, which ou the Young-Helmholtz theory should fall 

 into three, definite classes ; the peculiar and characteristic mistakes 

 of hue perception made by the colour-blind, detailed at length in 

 the book — all are irreconcilable with the three-colour theory of 

 vision. Of course, by letting the form of curve chosen to represent 

 each of the three sensations vary from case to case, and by making 

 the nature of the curve complicated enough, the theory might he 

 adapted to meet some of these objections, but in the process of 

 adaptation it would lose all its force. Dr. Edridge-Greens own 

 theory of vision falls into two main parts — one dealing with the 

 machinery of reception, and the other with the machinery of 

 perception. As regards the retina, he believes that the cones 

 alone receive and transmit the message, but they are sensitized 

 bv the visual purple secreted by the rods, and liberated into the 

 retinal fluid in response to light stimulation. Unless so sensitized 

 the cones are irresponsive. This very simple hypothesis explains 

 admirably all the peculiarities of foveal vision, and a whole series 

 of observations, including remarkable entoptic phenomena, for 

 which we must refer the reader to the book. As regards per- 

 ception, the author holds that the power of distinguishing colours 

 is one that has evolved from the power of distinguishing light 

 alone up to our present recognition of seven colours. He has 

 shown that a person of acute vision maps out twenty-seven 

 patches in the spectrum, each of which appears to him to be 

 monochromatic, and he regards this number as expressive of the 

 efficiency of the colour-perceiving centres of the brain. The less 

 developed these centres the smaller the number of " perceptively " 

 monochromatic patches, and the more colour-blind the person. 

 Some of these patches will, of course, in all cases be named as of 

 the same colour as their neighbours, though differing in hue. 

 Dr. Edridge-Green in his classification of the colour-blind dis- 

 tinguishes the heptachromic, whose colour perception is acute 

 and who sees seven colours in the normal spectrum ; the hexa- 

 chromic, who sees six colours ; pentachromic, who sees five ; and 

 so on down to the dichromic, the worst class of colour-blindness 

 usually met with, one variety of whom see the spectrum as blue 

 oue end and red the other, with a grey band in the middle. He 

 does not attempt to explain the precise mechanism of differentiation. 

 Dr. Houstoun, however, has recently put forward an interesting 

 mathematical theory which has led him to results in harmony with 

 Dr. Edridge-Green's view. 



Dr. Edridge-Green's theories seem to us to accord better than 

 any so far put forward with the present knowledge of the subject. 

 Whatever may be the ultimate judgment on this point, there can 

 be no question as to the value of his experiments and observations. 



