Literary Notices. 537 



often accepted oppositional relation between red-green and yellow-blue 

 owes its existence to aught inherent in the psychological analysis 

 itself. Similar doubt besets other such conclusions. Considering, then, 

 the uncertain charater of even the psychological inferences drawn 

 from an analysis of the sensations, it is far more precarious to draw 

 conclusions as to the physiological processes involved. The author's 

 general attitude towards the whole question, aside from the specific case 

 just referred to, is given in his introductory paper, already briefly men- 

 tioned, in connection with his discussion of the theory of specific compar- 

 isons. The point discussed is the validity of assuming, from the possibili- 

 ty of making comparisons of similarity between sensations unlike in qual- 

 ity (e.g. similarity in brightness of two different colors) the existence of 

 identical psychological or pliysiological elements which determine such 

 similarity. "If a physiological or a pyschological theory leads us 

 to assume in all [light] sensations a definite element, which determines 

 our impressions of brightness, then we may, on the basis of the theo- 

 ry, correctly refer to the equality or inequality of that element; but we 

 may not, conversely, deduce from the possibility of a brightness- 

 comparison the existence of such an element" (p. 28). The insistence 

 is, it is clear, so far as physiological inference is concerned, on the use 

 of only those physiological concepts in explanation which result logically 

 from a physiological theory based on recognized physiological proced- 

 ure (e.g. the response of a sense organ to its adequate stimuli), and 

 not on detached psychological analysis. 



The second point that I wished to bring forward, — v. Kries's 

 estimate of the achievements of research in visual sensation, may be 

 best stated in his own language : "If, inclosing, we summarize how 

 far theoretical concepts make it possible for us to elucidate and inter- 

 pret the facts, and indicate, in general, the state of our problem, we 

 may perhaps say, as of chief importance, that the view which we have 

 characterized as 'Duplizitatstheorie' explains in a wholly satisfactory 

 way a large number of functionally related phenomena This theory 

 regards the rods, containing visual purple, as the organs of 'twilight 

 vision,' and the cones as mediating a relatively different mode of vis- 

 ion, which we have called 'day vision.' One may further consider, 

 in the light of a zone theory [Zonentheorie], the elements that medi- 

 ate day vision as composed in their peripheral segments of red, green 

 and violet components, on the relative degrees of activity of which the 

 character of the sensations in part depends ; in part, however, on other 

 [more central] factors, which lead us to assume special conditions of 

 color vision [Farbigkeit], to distinguish, namely, on one hand a red-green 



