328 SCIENCE PROGRESS 



there has been no easily accessible general statement of them. The three- 

 colour theory of Young, Maxwell, and Helmholtz has naturally received such 

 prestige from the distinguished names connected with it that it has been 

 slow to weaken its hold, even though no anatomist has ever found a trace 

 of the three different retinal or nervous elements which it would seem to 

 require. The experimental evidence against it brought forward by Dr. 

 Edridge-Green goes far to render it quite untenable, or to demand such 

 modifications as would render it too complicated to be of any use, just as 

 the modifications which were introduced into the emission theory of light, 

 to make it accord with the facts, destroyed all its force. For details we must 

 refer the reader to the book : we can only mention the convincing proofs 

 of the simplicity of the yellow sensation, and the various cases of colour- 

 blindness detailed — such as the shortening of the red end of the spectrum 

 as a distinct defect — which cannot be reconciled with the hypothesis of a 

 missing, or diminished, green, red, or blue sensation. The author's own 

 theory, based on a study of colour-blindness unrivalled in the number of 

 cases examined, is that there are certain colour-perceiving centres in the 

 brain which have been gradually evolved from a state in which only a sensa- 

 tion of light could be perceived. It is a diminished power of distinguishing 

 between neighbouring colours in the spectrum which constitutes colour- 

 blindness. The optic nerve can convey any mixed type of vibration, but 

 the analytical centres differ in the power of discrimination in different in- 

 dividuals. Dr. Edridge-Green classifies observers as heptachromics, who 

 see seven distinct colours in the spectrum ; hexachromics, who see six ; 

 and so on, down to the dichromics, who see only two. (The totally colour- 

 blind see, of course, only a grey band varying in luminosity from point to 

 point). He himself has not attempted any physical explanation of the 

 nature of the nervous impulses, but recently Dr. Houston has written some 

 very suggestive papers on a mathematical theory of colour-perception, in 

 which he strongly supports Dr. Edridge-Green's views. 



Testing people for colour-blindness has a great practical importance in 

 the case of signalmen and lookout-men of all kinds, to mention only the 

 most obvious instance. Dr. Edridge-Green, having shown that the old 

 " wool test " is worthless, has devoted many years to the perfection of other 

 tests, and has had the satisfaction of seeing his methods adopted by the 

 Admiralty, the Board of Trade, and other bodies of similar status. He 

 has hitherto relied chiefly on his lantern, which is an expensive piece of 

 apparatus, but has now issued a card test which can be carried in the pocket, 

 and which promises to become the standard test where portability and 

 simplicity in operation are demanded. The test is fully described in the 

 book. The many ingenious features which it embodies are the fruit of a 

 prolonged experience — only those who have tried the older tests can realise 

 how defective they were — and have successfully eliminated all possibility 

 of the examinee using differences of form, luminosity, or other aids in read- 

 ing them. He must rely on his colour-perception alone. We have found 

 it a convincing and rapid test, which readily eliminates tetrachromics and 

 those of inferior grades of colour-vision, and we can confidently recommend 

 it to all interested in testing for colour-blindness. 



E. N. da C. A. 



ENGINEERING 



The Propagation of Electric Currents in Telegraph and Telephone Conductors. 



By Prof. J. A. Fleming, M.A., D.Sc, F.R.S. Third Edition. 

 [Pp. xiv -|- 370, with 97 diagrams and illustrations.] (London : 

 Constable & Co., 1919. Price 21s. net.) 



The appearance of the third edition of Dr. Fleming's book on the Propagation 

 of Electric Currents in Telephone Conductors is a welcome addition to the class 



