THE LOCAL QUANTITATIVE EFFECT 125 



and blue (B), A becomes green and brighter, B lilac and darker. 

 These changes are probably due to macular absorption. Tschermak^, 

 for example, was unable to distinguish definite variations in parafoveal 

 matches on increasing the eccentricity of fixation, thus confirming 

 earlier results by Hess and v. Kries. Under such conditions, however, 

 he found slight changes on increasing the size of the area stimulated, 

 and these were of the same kind as those found by Hering. 



The effect of the size of the area stimulated on the photochromatic 

 interval at the periphery and on the size of the field of vision has already 

 been discussed (p. 79). 



CHAPTER II 



SIMULTANEOUS CONTRAST OR SPATIAL INDUCTION 



We have seen (Section V, Chap, ii) that stimulation of a retinal area is 

 followed at a certain interval by sensations, potential or kinetic, of an 

 opposite nature, a phenomenon known as " succedaneous " or " succes- 

 sive contrast." There are numerous facts which tend to show that the 

 sensation resulting from stimulation of a retinal area is modified by 

 the condition of the surrounding areas, and per contra that stimulation 

 of a retinal area itself causes a change in the condition of excitability 

 of the surrounding area. This reciprocal interaction of retinal areas, 

 or, in terms less open to criticism, of contiguous or nearly contiguous 

 areas in the field of visual sensations is called " simultaneous contrast." 

 The term contrast (Chevreul) in each case indicates opponent activities. 

 Less open to objection is the term " induction " (Briicke), though this 

 term has unfortunately been used in a more restricted sense by Hering. 

 He uses " simultaneous induction " to connote change during fixation, 

 " successive induction," change occurring after removal of stimulation. 

 We shall use " induction " in the broadest sense, temporal induction 

 being any change in a visual area due directly to stimulation (and there- 

 fore including adaptation), and spatial induction being any change in 

 other areas resulting from the primary stimulation. To avoid circum- 

 locution we shall speak of " retinal " areas, without prejudice as to the 

 exact site at which the change manifests itself. 



Spatial induction manifests itself by changes both in luminosity 

 (or brightness) and hue. A patch of grey paper on a white background 



1 Arch. f. d. ges. Physiol. Lxxxn. 559, 1900. 



