536 Journal of Comparative Neurology and Psychology. 



that are not wholly alike, "but between which, in addition to a definite 

 difference, a certain similarity exists" (p. 22). 



The first two articles on vision proper, dioptrics (Schenck), and 

 the influence of light on the Retina (Nagel), bring up to date what is 

 known about these more objective phenomena of physiological optics. 

 A colored plate, in the latter, shows well the movements of the cones 

 and the pigment of the retina. 



Nearly two thirds of the present half-volume are occupied by v. 

 Kries's suggestive and subtle contribution to visual sensation. The 

 essay is subdivided as follows: laws of color-mixture, visual sensations 

 and their psychological ordering, dichromatic color systems, adapta- 

 tion of the visual organ, ecceutric vision, after-images, local and color 

 changes in the disposition (Urastimmung) of the visual organ, tempor- 

 al relations of the effects of light, induced light and color sensations, 

 limits of perception and discrimination, modifications in the color 

 sense brought about experimentally or by disease, effects of inadequate 

 stimuli, review of the facts and the results considered with reference to 

 the theoretical conception of the visual organ. 



It is possible to consider here only two points, — the author's gen- 

 eral methodological views, and his estimate of present achievements 

 in fact and theory. 



As to the first point, it is evident that at almost every step the au- 

 thor feels the necessity of separating sharply the psychological meth- 

 od, "for which the mutual relations of the sensations are of chief im- 

 portance, their dependence on stimuli, however, of lesser signifi- 

 cance" (p. 109), from the physiological method, for which the para- 

 mount problem is the relation of the reactions of the visual apparatus, 

 as indicated by the sensations, to its adequate stimuli, and of the 

 resulting facts to the objectively demonstrable properties of the visual 

 organs. An instance of the latter method is, of course, the attempt 

 to refer "day-vision" to the cones, and "twilight vision" to the rods. 

 V. Kries admits the possibility of a psychological analysis of the sen- 

 sations, but, since he questions certain current psychological inferences 

 drawn from such analysis, and condemns attempts to reach, through a 

 psychological ordering of the sensations, conclusions concerning the 

 physiological processes involved, he is skeptical as to the value of the 

 analysis. It is possible, for instance, with Aubert and Hering, to 

 pick out red, yellow, green and blue as four colors having no mutual 

 resemblances in quahty, but "an unbiassed person would hardly select 

 blue in complying with a request to designate a sensation which is the 

 contrary of yellow" (p, 135). It is doubtful, namely, whether the 



