576 



NATURE 



[July 8, 1920 



it should be reduced in intensity. Again, the eye 

 may be fatigued with red or green without alter- 

 ing the hue of spectral yellow. 



If the image of a white object be suddenly 

 formed on a portion of the retina which was pre- 

 viously occupied by the image of a black object, 

 this image is surrounded by a red border. If, 

 instead of white, a spectral greenish-yellow 

 illumination is used, the border is colourless ; if 

 the same greenish-yellow be made up of red and 

 green, it appears red (Bidwell). 



Many dichromics have a luminosity curve 

 similar to the normal, although their colour sensa- 

 tions are limited to red and blue at the ends of 

 the spectrum, with a neutral colour in between. 

 This would not be the case if their blindness were 

 due to the absence of one of the sensory units 

 (green). 



The theory which Dr. Edridge-Green has de- 

 veloped may be outlined as follows : — 



A ray of light impinging on the retina liberates 

 the visual purple from the rods, and a " photo- 

 graph " is formed. 



The ends of the cones are stimulated through 

 the photochemical decomposition of the visual 

 purple by light, and a visual impulse is set up 

 which is conveyed by the optic nerve to the brain. 



Instead of analysing this impulse into three 

 components. Dr. Edridge-Green regards it as an 

 integral unit the shape of which depends upon the 

 nature of the light exciting it. 



The physicist may be reminded that he himself 

 has already recognised that if the motions in the 

 aether corresponding to white light could be seen, 

 he would not be tempted to speak of them as 

 periodic, though they are capable of being re- 

 solved by Fourier's theorem into monochromatic 

 components. The gist of Dr. Edridge-Green 's 

 theory is that he deals with the visual impulse as a 

 unit, but asserts (in effect) that if for convenience 

 it is resolved into components, the number of 

 necessary components is usually large. We do not 

 mean that he says this in so many words ; but 

 this is, in physical language, what his statements 

 appear to us to imply. His theory is therefore 

 of greater generality than the restricted Young- 

 Helmholtz theory which it supplants. 



(2) The card test, which is supplementary to the 

 text-book, consists of twenty-four cards, each con- 

 taining a large number of irregular, coloured 

 patches or spots. The shapes of these are pre- 

 cisely the same on all the cards, so that the 

 examinee cannot be coached to discriminate by 

 the form alone. These patches are differently 

 coloured on all the cards. Each card contains a 

 number of patches of a selected . hue, different 

 from the other patches, arranged in the form of 

 NO. 2645, VOL. 105] 



a letter. The examinee is required to declare 

 the letter on each card in turn. The colours are 

 so chosen as to enable the examiner to discrim- 

 inate between the different kinds of colour blind- 

 ness. 



We have tested them on numerous individuals. 

 Card 8 is particularly useful in the quick detection 

 of weakness in the green. It contains a green 

 C and a brown S. To a normal individual the 

 C i^ very prominent, while the S is a difficult 

 letter to detect. One examinee who was quite 

 unconscious that he was in any way defective 

 detected the S instantly, while he could not detect 

 the C even when his attention was directed to it. 



We have not space to discuss either the book 

 or the card test fully. We congratulate Dr. 

 Edridge-Green on having brought together a 

 wealth of important and interesting material on 

 the physiology of vision. 



Hydrographical Surveying. 



Hydro graphical Surveying: A Description of 

 Means and Methods Employed in Constructing 

 Marine Charts. By the late Rear-Admiral Sir 

 William J. L. Wharton. Fourth edition, revised 

 and enlarged by Admiral Sir Mostyn Field. 

 Pp. xii-l-570. (London: John Murray, 1920.) 

 Price 30s. net. 



THE fourth edition of this work on hydro- 

 graphical surveying differs but slightly from 

 its predecessor, the main text being practically 

 untouched, and the only important changes being 

 the addition of several articles on newer surveying 

 methods and experimental devices which had been 

 introduced in the years immediately preceding the 

 war. 



Of these the description of a form of "vacuum 

 tide-gauge," devised by Rear-Admiral H. E. 

 Purey-Cust, a former Hydrographer of the Navy, 

 is perhaps the most interesting, and it will cer- 

 tainly appeal to every nautical surveyor who has 

 had to fight against the difficulties of observing 

 the vertical movements of the tide in situations 

 where direct readings are almost impossible. The 

 addition of a trustworthy self-recorder to the in- 

 strument is obviously merely a question of time 

 and experiment, and when it has been perfected 

 this form of tide-gauge will undoubtedly prove an 

 immense boon to nautical surveyors for use in 

 those parts of the world where the ordinary 

 methods of tide reading are impracticable. 



It is to be regretted that no mention has been 

 made of the extremely useful and convenient form 

 of current meter known as the "Ekman.'" This 

 instrument has been used with conspicuous success 



