SIMULTANEOUS CONTRAST OR SPATIAL INDUCTION 127 



extent of the background, as well as its luminosity and colour, modifies 

 the luminosity and saturation of the contrast colour. 



The contrast effect is not uniform over the whole surface, but is 

 most marked at the edges. A distinction is therefore sometimes made 

 between " surface-contrast " {Fldchenkontrast) and "border-contrast" 

 {Randkontrast). The disparity diminishes on prolonged fixation (" simul- 

 taneous induction," Hering). Border contrast is easily demonstrated 

 with black and white, or coloured sectors on a rotating disc (Mach^, 

 Aubert, v. Helmholtz), or by the shadow method (Goethe, A. Fick^). 

 It is visible by momentary stimulation and is reversed in the negative 

 after-image (Mach) : in the latter case it may be more obvious than in 

 the primary image. With colour-contrast the saturation is greater at 

 the borders than in the middle. 



Contrast effects are diminished by separation of the contrasting 

 fields, and are therefore seen better with small than large fields. A 

 narrow black line between the fields diminishes contrast markedly 

 (v. Helmholtz), and the effects are greatest at contiguous contours 

 (Mach). 



Jurin (1783) and Brandes (1827) attempted to explain simultaneous 

 contrast by successive contrast, due to slight movements of the eyes. 

 Though this factor is a frequent complication and requires special heed 

 to its elimination it is not the cause (Fechner^, Hering). With the 

 greatest care in controlling the movements of the eyes and keeping 

 accurate fixation, simultaneous contrast manifests itself immediately. 

 Moreover it occurs with colours of such weak saturation that no 

 coloured after-image is induced. Aubert, Mach, Meyer'* and Rollet^ 

 have shown that it occurs on momentary stimulation. 



Attempts have been made to obtain quantitative measurements of 

 simultaneous contrast by means of a comparison field by Lehmann^, 

 Ebbinghaus'^, Hess and Pretoria and others. 



For black and white or pure luminosity (brightness) contrast 

 Ebbinghaus found that the increase in brightness of a patch on a darker 

 background is proportional to the difference of the two light intensities, 

 irrespective of their absolute values. Hess and Pretori found that the 

 apparent luminosity of a small bright surface on a dark background 



^ Sitz. d. Wiener Aknd. Lii. 2, 303, 1865. 



2 Hermann's Handh. d. Physiol, iii. 1, 3, 1879. 



3 Ann. d. Physik xliv. 513, 1838 ; L. 193, 427, 1840. 



^ Amer. J. of Sc. XLVi. 1, 1893. ^ Sitz. d. Wiener Akad. lv. 424, 1867. 



« Wundt's Philos. Stud. iii. 497, 1886. 



' Sitz. d. Aknd. d. Wiss. Berlin, 994, 1887. » Arch. f. Ophth. xl. 4, 1, 1894. 



