128 COLOUR VISION 



remains constant when the increase of illumination of the surface 

 bears a constant ratio to the increase of illumination of the background. 

 The contrast effect therefore varies directly with the inducing stimulus 

 and is independent of its absolute value. Ebbinghaus and Hess and 

 Pretori agree therefore as to increase of brightness by contrast or 

 contrast-brightening. Hess and Pretori find that the law also applies 

 to contrast-darkening, therein differing from Ebbinghaus, but the results 

 of the latter are probably due to error in technique. Kirschmann 1 

 found that the amount of contrast varies as the square root of the area 

 of the inducing field, i.e., the relationship between the area and the 

 intensity of the inducing light is reciprocal. 



For colour-contrast Fechner showed that the brightness of a grey 

 background must be appropriately chosen. Under optimum conditions 

 an inducing colour of very slight saturation suffices to induce marked 

 contrast. The statement, however, that simultaneous contrast is most 

 evident with slight differences of colour between the reacting fields 

 (v. Helmholtz) is not true (Fechner, Rollet). Hering 2 showed that 

 with optimum black- white background conditions contrast increases 

 with the saturation of the coloured inducing field, an observation 

 confirmed by Pretori and Sachs 3 . Kirschmann (1892) had previously 

 arrived at the same result, but found that the increase was not 

 proportional, but probably a logarithmic function. Colour-contrast, 

 as might be expected, is accompanied by change in luminosity, and 

 this subject has been exhaustively studied by Pretori and Sachs under 

 Hering's guidance. Their results will be considered in Part III. 



The most important element in colour-contrast from our present 

 point of view is the opponent or complementary effect, early noticed 

 by Briicke 4 . Under ordinary circumstances the induced colour is not 

 the precise complementary of the inducing, as was known to Goethe, 

 and has been fully investigated by Hering 5 . Various physical factors 

 conduce to the effect, such as the variable nature of "white" light 

 and " grey " surfaces, macular pigmentation (Hering, Sachs), the pig- 

 ment of the lens (Hering), the brown pigment of the retinal epithelium 

 (Tschermak), and the reddening by the blood of the light which passes 

 through the sclerotic (Hering, Hess). These do not suffice to explain 

 all the facts. There are other, physiological, factors, such as the 



1 Wundt's Philos. Stud. \i. 417, 1890 ; vn. 362, 1891 ; Amer. J. of Psychol. iv. 4, 74, 

 542, 1892. 



2 Arch.f. d. ges. Physiol. XLI. 1, 1887 ; XLII. 117, 1888. 



3 Ibid. LX. 71, 1895. 4 Ann. d. Physik, LXXXIV. 418, 1851. 

 8 Zur Lehre vom Lichtsinne, 1876. 



