538 Journal of Comparative Neurology and Psychology. 



and on the other a yellow-blue sense. With these assumptions one may- 

 take account not, to be sure, of all, but nevertheless of a very great 

 number of the known facts. We may, indeed, represent the protan- 

 opic [red-blind] and the deuteranopic [green-blind] visual organs as 

 originating in a lack of the red and green components respectively, 

 the 'rotanomale' and the 'griinanomale' in a variation in the nature 

 of the red or the green component, and the color blindness of the 

 eccentric regions of the retina, as well as acquired color blindness, in 

 a lack of the more centrally conditioned red-green and yellow-blue 

 senses; we are then in a position to represent simply the large number 

 of facts that are found in the vision of these various individuals, or of 

 the various parts of the visual organ, and to account for them, with 

 simple presuppositions, in a way throughout and exactly (so far as we 

 can say) in accord with experience." 



To exhaust the significance of this passage would require a synop- 

 sis of the whole article. The essence of the "Duplizitatstheorie" (for 

 which it is hard to find a good translation) is, however, that the retina 

 is a twofold visual apparatus, the rods, capable of "dark-adaptation," 

 mediating colorless light in the presence of weak stimuli (twilight vis- 

 ion), and the cones, scarcely influenced by such adaptation, mediat- 

 ing our ordinary color (day) vision. This is perhaps the most widely 

 accepted assumption in visual sensation, and tC v. Kries is due the 

 chief credit of its experimental development. The zone theory assumes 

 the validity of the triple-component theory (Young-Helmholtz) for 

 peripherally situated visual elements, the possibility, however, in order 

 to explain certain facts referred to in the quoted passage, of a four- 

 fold division of elements, which form the basis of Aubert's and Her- 

 ing's thought, for the central [brain] portions of the visual apparatus. 



Since the article is in no sense a defence or a criticism of any ex- 

 clusive theory, but rather a calm attempt to do justice, in both fact and 

 theory, to the various and perplexing results of accomplished research, 

 it would be inept, in this short review, to attempt to epitomize the ar- 

 guments for or against the various theories. One feels, however, that 

 the effect of v. Kries's present contribution will be to win still more 

 serious consideration for Helmholtz's general point of view, and to 

 raise further questionings as to whether Bering's theory can so legit- 

 imately account for the manifold facts as is often assumed. One awaits 

 therefore, with considerable interest, the appearance of Herings' pro- 

 nounced volume for "Graefe-Saemisch's Handbuch." 



It is a pleasure to read v. Kries; for, while the style is often involv- 

 ed (at least for a foreigner) and occasionally abstract, the exposition 



