August 6, 1891] 



NA TURE 



315 



Now, Hering^ has particularly investigated this portion 

 of the subject, and explains the existence of two forms of 

 colour-blindness as follows. He finds that persons with 

 a normal colour-vision can be divided into two groups. 

 The one class perceive yellow, the other blue, with ex- 

 ceptional ease, probably owing to a difference in the pig- 

 mentation of the media of the eye. The difference 

 between the two groups is best seen with spectral green ; 

 for a green can be found which appears at the same time 

 yellow-green to the one, blue-green to the other. To an 

 observer with strong yellow vision, almost the whole of 

 spectral red appears to be tinged with yellow, whilst a 

 member of the second group, whose strong sense of blue 

 prevents his seeing the yellow, pronounces the greater 

 part to be pure red. Thus, the pure red and the pure 

 blue are radically different colours for the two groups. 

 Now, it is found that the pure red and the pure green 

 formed for an observer with a strong sense of yellow 

 appear grey to one who is what is called green-blind ; 

 whilst, on the other hand, the pure red or the pure green 

 of the observer with a strong blue sense appears colourless 

 to one who is red-blind. A red which is invisible to one 

 who is "red-blind" is evidently coloured to a patient 

 who is green blind, and he speaks of the colour he sees 

 as red. But if a minute proportion of blue is added, the 

 red gradually becomes purer until it becomes free from 

 yellow to those of us who have a strong yellow sense. 

 As the red becomes purer, our green-blind patient com- 

 plains that the " red" is fading, and when finally the red 

 is quite pure he matches the colour he sees with a grey, 

 and says that the colour has gone. Thus, there is no 

 fundamental difference between the red- and the green- 

 blind. Neither group can perceive red or green. The 

 only difference between them is one which we find 

 amongst normal-sighted persons— namely, a different 

 visual acuity for yellow and blue. The "red" of the 

 green-blind is in reality the secondary sensation of yellow 

 yielded by almost all the reds in nature, differing from 

 the ordinary yellow in its limited power of exciting white. 

 This peculiar yellow he has learnt to associate with what 

 others around him call red, and he only betrays his afiflic- 

 tion when all yellow is eliminated from the colour he calls 

 red. Thus, a consideration of colour-blindness again 

 leads us to throw red and green, blue and yellow, 

 together into two groups as primary colour sensations. 



Simultaneous contrast is touched on very superficially, 

 and successive contrast is scarcely mentioned, yet the 

 author again grasps at the three-colour theory to explain 

 the few phenomena he mentions. Yet it is notorious 

 that the Young-Helmholtz theory fails to afford any 

 adequate explanation of the phenomena of contrast. It 

 was by an ingenious contrast experiment that Hering 

 produced such a striking confirmation of his views before 

 the Physiological Congress at Basle, and placed the 

 three-colour theory in a dilemma from which its ablest 

 exponents have not yet succeeded in extracting it. 



In conclusion, the book before us is an admirable 

 summary of a valuable series of experiments. We can 

 scarcely imagine that it will appeal to the public in 



' "Zur Erkliirung d. Farbenblindheit " (Prag, 1880); " Ueber Indi- 

 viduelle Verschiedenheiten des Farbensinnes " (Prag, 1885); " Eine 

 Vornchtung z. diagnose d. Farbenblindheit," "Ueber d. Erklarung d. 

 penpharen Farbenblindheit," " Einseitige StSrungen d. Farbensinnes," 

 Archivf. Ophthalmologie, xxxvi. 



NO. I 136, VOL. 44] 



general. But it should be read by those who are in- 

 terested in the phenomenon of colour-vision, and the fact 

 that the author frankly accepts the three-colour theory 

 and ignores the work of Hering does not, in our opinion, 

 detract from its value. For the book thus becomes an 

 admirable statement of the strongest portion of the physi- 

 cal theory of colour by one of the ablest of English 

 physicists. H. H. 



POSITIVE SCIENCE AND THE SPHINX. 

 Riddles of the Sphinx. A Study in the Philosophy of 

 Evolution. By a Troglodyte. (London : Swan 

 Sonnenschein, 1891.) 



THESE be old old riddles that the Sphinx propounds 

 and the Troglodyte attempts to guess, in the volume 

 before us ; none other, indeed, than the What, Whence, 

 and Whither of man and of the world. There have been 

 other guesses in the past, there will be other guesses 

 while time lasts ; each guesser thinks his own guess 

 nearer the true answer than any other ; his neighbours 

 mostly smile, unless his guess chances to be something 

 like their own ; and the Sphinx looks on with stony stare, 

 imperturbable, giving no hints. 



So soon as man, as man, looked out upon the world, 

 and began dimly to realize the first personal pronoun, the 

 nascent reason, or, if the phrase be preferred, intellectual 

 faculty, demanded, for the first time in the history of the 

 development of consciousness, an explanation. Man, 

 then as now the chief centre of interest to man, must 

 thenceforward not only live and act, but must seek to 

 explain his life, and his activity. Yesterday the tribe- 

 chief went forth a living man, feared by all : to-day his 

 body is brought back, helpless, lifeless, and a hog spurns 

 it with his snout. How account for this.? How explain 

 this change.? Something there was about the man 

 yesterday which made him totally different from the 

 mere mass of clay that to-day already needs hustling out 

 of sight. That something, call it soul, spirit, energy, life, 

 what you will, has departed. Whither has it gone .? 



This question, eminently natural, almost inevitable, 

 opened the way for reason's first blunder to enter and to 

 become a fruitful mother of children. Reason, in the 

 exercise of the new-born analytic faculty, distinguished 

 between the mere body and the informing something 

 through which it was a living body ; between the material 

 substance and the spirit-energy which was associated 

 with that substance during life. But reason also jumped 

 to the conclusion that what were distinguishable in 

 thought were also capable of separate existence in fact. 

 The matter remains in the corpse, but the something, 

 the spirit-energy, has escaped, to lead a distinct and in- 

 dependent existence. In justification of this conclusion 

 the phenomena of dreams were no doubt adduced as 

 evidence. While the chief's body was lying stark and 

 stiff, his true self, his spirit-energy, appeared by night to 

 more than one of his chosen followers. Thus the dream 

 seemed to support the false conclusion of the nascent 

 reason, which had not yet learnt to distinguish without 

 dividing. 



It has cost positive science much labour, and not a 

 few hard blows, to establish, by detailed work in physical 

 science, biology, neurology, and psychology, the ille- 



