REVIEWS . 327 



The diagrams illustrating points of technique are clear, and the text is 

 concise and to the point. As an aid to the practitioner who, whilst appre- 

 ciating the value of accurate pathological diagnosis, cannot find the time 

 to carry out the technique himself, this guide should prove very useful. 



H. A. Haig. 



The Physiology of Vision, with Special Reference to Colour- Blindness. By 



F. W. Edridge-Green. [Pp. xii + 280, with 23 figures.] (London : 



G. Bell & Sons. Price 12s. net.) 



Card Test for Colour-Blindness. By F. W. Edridge-Green. (G. Bell & 

 Sons. 25s. net.) 



The subject of colour- vision has engaged the attention of physicists and 

 physiologists from the time of Boyle to the present day. Newton himself, 

 who paved the way for all subsequent researches in colour from the phy- 

 sicist's point of view, was not very keenly interested in the question of 

 colour mixing : " For I could never yet by mixing only two primary 

 Colours produce a perfect white. Whether it may be compounded of a 

 mixture of three ... I do not know, but of four or five I do not much ques- 

 tion but it may. But these are curiosities of little or no moment to the 

 understanding the Phsenomena of nature." In other words, the way in 

 which the eye interprets the physical facts was for him a minor matter. 

 Two physicists only less great than him, however, Thomas Young and 

 Helmholtz, both men of wide medical and physiological knowledge, devoted 

 much study to the mechanism of vision, and the theory of colour perception 

 known by their joint names, suggested by the former and elaborated by 

 the latter, obtained much support. For the physicist the spectrum of 

 white light is a band of colour in which every line element corresponds to a 

 perfectly definite and measurable wave-length, the distinguishing of any 

 portion of which from its neighbour is merely a question of the resolving 

 power of the instrument. It is the task of the physiologist to solve the 

 very complicated problem of the relationship between wave-length — and 

 mixture of wave-lengths — and colour perceived, and to analyse the mechan- 

 ism by which the physically infinite number of colours is reduced to the 

 few simple sensations known as the primary colours. The phenomena 

 which he has to investigate are extremely varied, and besides ordinary 

 colour-perception, the complex subjects of simultaneous and successive 

 colour contrast, positive and negative after images, colour adaptation and 

 other similar efiects, have to be studied. 



The book which is before us covers the whole field of physiological optics, 

 in nearly every department of which Dr. Edridge-Green has made striking 

 advances. His theory of the functioning of the retina is now generally 

 held to furnish the most comprehensive and suggestive explanation that 

 has yet been ofiered of the peculiarities of foveal and peripheral vision, the 

 movement of after-images, and various other phenomena. On this theory 

 the cones — and it will be remembered that the fovea contains cones only 

 — are the perceptive elements, but they are in a condition to convert the 

 incident light into nervous impulses only when the retinal fluid in which 

 they are bathed contains the visual purple. It is the function of the rods 

 to liberate this visual purple when stimulated by light A whole series of 

 entoptic phenomena which are in closest agreement with this view are now 

 known, and the features of foveal vision, such as the disappearance of a 

 small bright object, find in it an exact representation. 



The importance of Dr. Edridge-Green's work on colour vision is, in 

 spite of a somewhat unscrupulous opposition in the past, at length receiving 

 adequate recognition, and a wide circle of readers will welcome the summary 

 of his views on the subject contained in this book, especially as hitherto 



