182 SECTIONAL ADDRESSES. 



explanations O'f successive contrast as due to fatigue, and of simultaneou"? 

 contrast as due to psycliological factors. Eivers clearly showed that 

 the effect of psychological factors is not to create but to mask the 

 phenomena of simultaneous contrast, which are really dependent on 

 what he terms ' the physiological reciprocity of adjoining retinal areas.' 

 His enthusiasm for Hering's theories led him to give by far the most 

 detailed presentation of them that had then or has since appeared in 

 our language. In classifying the phenomena of red-green colour- 

 blindness, on which Helmholtz largely based his trichromic theory, 

 Eivers proposed the useful terms ' scoterythrous ' and ' photerythrous 

 in place of the terms ' protanopic ' and ' deuteranopic,' so as to avoid, 

 in describing these phenomena, the use of names which implied the 

 acceptance of a particular theory of colour vision. These terms have 

 failed, however, to obtain general adoption. 



In 1896 Rivers published an important paper ' On the Apparent Size 

 of Objects ' {Mind, N.S., vol. v., pp. 71-80), in which he described 

 his investigations into the effects of atropin and eserin on the size of 

 seen objects. He distinguished two kinds of micropsia which had 

 hitherto been confused — micropsia at the fixation-point due to irradia- 

 tion, and micropsia beyond the fixation-point, which is of special psy- 

 chological importance. Rivers came to the interesting conclusion that 

 the mere effort to carry out a movement of accommodation may produce 

 the same micropsia as when that effort is actually followed by movement. 

 In other words, an illusion of size may be dependent solely on central 

 factors. His later work, in conjunction with Professor Dawes Hicks, 

 on 'The Illusion of Compared Horizontal and Vertical Lines,' which 

 was pubUshed in 1908 {Brit. J. of Psychol., vol. ii., pp. 241-60), led 

 him to trace this illusion to origins still less motor in nature. Here 

 horizontal and vertical lines were compared under tachistoscopic and 

 under prolonged exposure. The momentary view of the lines in the 

 tachistoscope precluded any m.ovement or effort of movement of the 

 eyes, which had been supposed by many to be responsible for the over- 

 estimation of vertical lines owing to the greater difficulty of eye move- 

 ment in the vertical as compared with the horizontal direction. The 

 amount of the illusion was found to be approximately the same for 

 tachistoscopic as for prolonged exposure of the lines, but in the tachisto- 

 scopic exposure the judgment was more definite and less hesitating — in 

 other words, more naive, more purely sensory, more ' physiological ' — 

 than in prolonged exposure. This result, which led to further work 

 by Dr. E. O. Lewis at Cambridge under Rivers upon the Miiller-Lyer 

 illusion and upon the comparison of ' filled ' and ' empty ' space, is of 

 fundamental psychological importance. Although it is not inconsistent 

 with the view" that visual space perception depends for its genesis on 

 eye movement, it compels us to admit that visual space perception, 

 once acquired, can occur in the absence of eye movement; or, in more 

 general language, that changes in consciousness, originally arising in 

 connection with muscular activity, may later occur in the absence of 

 that activity. The provision of experimental evidence in favour of 

 so fundamental and wide-reaching a view is obviously of the greatest 

 importance. 



