July 8, 1920] 



NATURF 



575 



professors of the clinical subjects, for the endow- 

 ment of which it gave 250,000^, a quarter of 

 the cost. In 191 7 it gave the Chicago Uni- 

 versity 500,oooL, and in ninety days the Univer- 

 sity collected a further 90o,oooL to complete the 

 endowment of full-time clinical chairs. In 1918 

 Yale University raised 650,000^. for the same 

 purpose, of which the Rockefeller Foundation con- 

 tributed one-quarter. In 1919 the Johns Hopkins 

 Hospital established a full-time teaching staff in 

 obstetrics and gynaecology, with an endowment 

 of 250,000/., of which the Rockefeller Foundation 

 gave ioo,oooL It is rumoured that the same 

 Foundation, which has also given such vast en- 

 dowments for medical education in Canada and 

 China, is about to excel all its former efforts by a 

 new scheme for further helping medical education 

 in the United States. With such examples of the 

 scale on which these things have to be done, surely 

 England can do more for medical education than 

 she is doing ! 



The task of the reformer of medical education is 

 vastly more difficult in this country than in 

 America, because on every side there is the hamper- 

 ing influence of cast-iron conventions ; but now that 

 the Rockefeller Foundation has helped us to begin 

 the urgent reform there can be no doubt as to the 

 ultimate result. 



The Theory and Facts of Colour Vision. 



(i) The Physiology of Vision, with Special Refer- 

 ence to Colour Blindness. Bv Dr. F. W. 

 Edridge-Green. Pp. xii -1-280, (London: 

 G. Bell and Sons, Ltd., 1920.) Price 125. net. 



(2) Card Test for Colour Blindness. By Dr. F. W. 

 Edridge-Green. 24 cards. (London : G. Bell 

 and Sons, Ltd., n.d.) Price 255. net. 



(rjT^HE great importance of the subject-matter 

 X of the volume under notice and of the card- 

 test which supplements it is beyond all question. 

 Interest in it is enhanced by the fact that the sub- 

 ject is admittedly full of difficulties. In every dis- 

 cussion of human sensations and of the organs 

 which serve as the receivers of stimuli, one is im- 

 pressed by the uncertainty of much which has been 

 put forward as assured truth. It is not long ago 

 that the mechanism of audition was being dis- 

 cussed anew, and even now, in spite of the re- 

 newed examination, the functions of various parts 

 of the ear are much in debate. Yet in audition 

 we have to deal with purely mechanical stimuli 

 which we might have expected to have yielded up 

 NO. 2645, VOL. 105] 



the secrets of their operation long ago. In the 

 case of light the problem is clearly of a more 

 recondite order, and it is not so surprising that 

 little is actually known with certainty about the 

 functions of various parts of the eye, and that we 

 have therefore to fall back upon surmise. 



The theory of vision most espoused by physi- 

 cists is the three-colour theory of Young and 

 Helmholtz, based upon the facts of colour mix- 

 ture. It is possible to reproduce any tint what- 

 ever by mixing together three selected tints in a 

 suitable proportion. This is accepted now by 

 every school, and it must be taken as the basis 

 of any theory of colour vision. The Young- 

 Helmholtz theory explains the fact by assuming 

 that there are three units in the sensitive ap- 

 paratus of the eye (either three sorts of nerves 

 or rods or cones), each of which responds 

 in a maximum degree to one of the three 

 primary tints, but also to a less degree to 

 all (or most) other tints. Red, green, and blue 

 of selected wave-lengths are taken (for reasons 

 which cannot be given here) as the primary tints. 

 A spectral yellow stimulates both red and green 

 sensations, so does a mixture of red and green 

 lights; hence a certain such mixture will produce 

 the same sensation as does a spectral yellow. In 

 this way the phenomena of colour mixture are 

 explained. 



Unfortunately, there are difficulties in accept- 

 ing this theory. In the first place, there is no 

 histological evidence of the existence of these 

 three units. This objection, taken alone, is not 

 fatal. It is conceivable that anatomical differ- 

 ences exist which are beyond detection with the 

 microscope. But, in addition, there is a vast 

 number of phenomena to be explained besides 

 those of colour mixture, and many of these seem 

 to be directly in opposition to the theory. Dr. 

 Edridge-Green is well known as one who, after 

 prolonged study of the question, was compelled 

 to give up the trichromatic theory. The volume 

 under review summarises the conclusions to which 

 he has come. We can cite only a few of the 

 experimental facts. 



In certain cases of defective colour perception 

 the yellow sensation is diminished, and in others 

 lost altogether, although the percipient ex- 

 periences three definite colour sensations (red, 

 green, and violet). Why do not the red and green 

 make yellow in such cases? If the eye be 

 fatigued with pure spectral yellow light, and be 

 then turned aside to view a spectrum, this will 

 appear to have lost its yellow ; and though 

 yellowish-red or yellowish-green will appear less 

 yellow, the terminal red of the spectrum will not 

 be affected. According to the trichromatic theory, 



