NATURE 



2b' 



THURSDAY, SEPTEMBER i, 1910. 



COLOlR-VlSlOy. 

 Cplour-Blindiiess and Colour-Perception. By Dr. 

 F. W. Edridi;;e-Green. Second edition. Pp. xii -r 

 -,i2. International Scientific Series. (London : 

 Kegan Paul, Trench, TrCibner and Co., Ltd., 1909.) 

 Price 55. 



DR. EDRIDGE-GREEX has been engaged for 

 more than twenty years in advocating opinions 

 about colour-blindness which he has not the gift of 

 stating with extreme lucidity, which most people find 

 it by no means easy to understand, and it is 

 therefore somewhat diflScult either to accept or to re- 

 fute. The general basis on which the opinions rest 

 appears to be his conception of something called by 

 him a " psycho-physicrd unit," by which he ap- 

 pears to mean the limit, in any individual, of the 

 power to perceive actual difference between two things 

 closely resembling one another, as, for example, 

 between two similar colours, or between two masses 

 of the same substance that are of nearly the same 

 magnitude. There are, no doubt, great personal 

 differences of this kind, differences which may be 

 partlv congenital and partly the results of training ; 

 but it does not appear to us that the prefix "psycho," 

 whatever it mav mean, bears any intelligible relation 

 to them. The differences are differences of the acute- 

 ness of sense-perception, and, if we regard simple 

 sense-perception, colour-perception, for example, as 

 "psvchical," we must postulate the activity of a 

 "psyche" in the humblest fly which is guided to the 

 nectary of a flower bv the colour of the corolla. 



We do not know whether Dr. Edridge-Green is 

 prepared for such an extension of the domain of 

 " psvchologv," and mav leave him to settle the ques- 

 tion with the professors of that branch of speculation, 

 but we mav admit that the condition known as 

 "colour-blindness" may fairly be said to consist of 

 an inabilitv to respond by accurate sense-perc:ption 

 to the impact of light-waves of certain amplitudes, and 

 of a consequent liabilitv either to ignore them com- 

 pletelv or to confound them with waves of other 

 amplitudes. It is at least highly probable that minute 

 differences of this kind are extremely common, and 

 Lord Rayleigh long ago showed that some persons, 

 whose colour-sense could only be described as normal, 

 nevertheless differed from others in respect of the pre- 

 cise admixtures of light from different portions of the 

 spectrum, more especially in respect of the admixtures 

 of red and green, which they were prepared to accept 

 as a perfect "match" for a test-spot given as a 

 standard. 



It is probable that such terms as "red-blind" or 

 "green-blind" might be extended not only to the six 

 classes (hexachromic, pentachromic, &x.), described by 

 Dr. Edridge-Green, but to a much larger number; 

 and, by the wav, we do not know on what ground 

 our author regards the normal-sighted as " hexa- 

 chromic " onlv, and so apparently excludes from the 

 spectrum, as thev see it, the seventh distinct colour, 

 indigo, which was described bv Newton, and has coni- 

 NO. 2 13 I, VOL. 84] 



monly been accepted by later observers. The limita- 

 tions of our space forbid us to follow these questions 

 into detail, and the chief practical importance of 

 colour-blindness depends upon the fact that a liability 

 to make mistakes about colour-signals involves 

 dangers to life and property on railways and in 

 navigation. 



As regards protection against these dangers, we do 

 not see that Dr. Edridge-Green has furnished us with 

 any increased security, or, indeed, that any better 

 security is needed than is obtained from Holmgren's 

 wool test, when this is employed in the precise manner 

 directed by its originator, whose very definite instruc- 

 tions are too often departed from. The elaborate 

 lanterns and slides described by Dr. Edridge-Green are 

 in all essential respects identical with many of the 

 tests used by the Royal Society's committee; and, if 

 we may admit that they afford means of distinguish- 

 ing one case of colour-blindness from another of a 

 slightly different type, we cannot admit that they are 

 calculated to afford any increased security to the in- 

 dustries in which the power to distinguish promptly 

 between different signal colours is required. 



A HISTORY OF BIOLOGICAL THEORIES. 

 Geschichte der biologischen Theorien. By Dr. Em. 

 R^dl. II. Teil. Geschichte der Entwicklungstheorien 

 in der Biologie des XIX. Jahrhunderts. Pp. x -1-604. 

 (Leipzig : \V. Engelmann, 1909.) Price 16 marks. 



THE author of this scholarly work attaches great 

 importance to the cultivation of the historical 

 sense among biologists, believing that progress is 

 impeded because there is relatively little of it, smd one 

 of the aims of his book is to stimulate a study of the 

 history of the science. To this end it is admirably 

 adapted. It is learned, but at ihe same time wisely 

 selective; it is at once appreciative and critical; and 

 it is written in a fresh, interesting way. We had the 

 pleasure of welcoming the first volume, published 

 four vears ago, w-hich dealt with ancient history, and 

 we would congratulate the author again on the success 

 with which he has accomplished a very difficult task 

 in dealing with what has occurred in biology, or in 

 biological Eetiology, since the end of the eighteenth 

 century. It seems to us, indeed, that the author has 

 added to his strength since he completed the first part 

 of his great work. 



There are forty-one chapters in the book, and we 

 may note some of the titles to suggest the range of 

 discussion :— Lamarck and Cuvier, idealistic mor- 

 phology, embryology before Darwin, the cell-theory, 

 physiology before Darwin, transition from Naturphilo- 

 sophie to modern science, origin of Darwinism, 

 Darwin, Wallace, reception of Darwin's theory, influ- 

 ence of Darwinism, Ilaeckel, spontaneous generation, 

 anthropology, Darwinian morphology and embryology, 

 geographical distribution, palaeontology, natural selec- 

 tion, heredity, psychology, Lamarckism, species, repro- 

 duction, "crossing, developmental mechanics, Driesch, 

 decline of Darwinism, the history of science. 



What the author aims at is a historical appreciation 

 of the significance of the various stages in the develop- 

 ment of aetiology, and this involves a critical judgment 



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