126 COLOUR VISION 



looks darker than a patch of the same paper on a black background 

 (Goethe). Innumerable experiments have been devised to demonstrate 

 luminosity (or brightness) contrast. Especially instructive are those 

 with Hering's double-room or double-screen method, in which an aperture 

 in a screen is variously illuminated from behind, whilst the illumination 

 on the front of the screen can also be varied independently at will. 



It has been recognised since the time of Leonardo da Vinci that 

 coloured surfaces are altered in hue by their surroundings, and vice versa. 

 Briicke called a coloured surface surrounded by white the inducing 

 colour. It must, however, be well borne in mind that the phenomenon 

 is reciprocal, no matter what the luminosity or hue of the two fields 

 may be. Tschermak^, who has written an excellent resume of our 

 knowledge of contrast, enumerates five methods of demonstrating colour 

 contrast. 



(1) The background method was early studied by Chevreul^. In 

 its simplest form white, grey, black, or coloured patches or strips are 

 laid on coloured backgrounds. The contrast is accentuated by placing 

 tissue-paper over both^. Maxwell's top can also be used (Dove, 

 V. Helmholtz, H. Meyer, Hering^, and others), or a projection apparatus 

 (Rollet). 



(2) The mirror method, used by Goethe, is familiar in Ragona 

 Scina's well-known experiment, which has been modified and improved 

 by Hering''^, 



(3) The method of coloured shadows dates from Goethe, and has 

 been much used, notably in recent years by Hering*" and Abney for the 

 detection of colour-blindness. 



(4) The double-image method was introduced by v. Helmholtz'^ 

 and modified by Hering^. 



(5) Negative after-images have also been used by Hering^. 



Chevreul, Fechner, and Wundt^° showed that actual contiguity of the 

 reacting surfaces was not essential. Aubert^^ showed that weak and 

 localised stimulation altered the excitability of the whole retina. The 



1 Ergebnisse d. Physiol, ii. 2, 726-798, 1903. 



^ De la Loi du Contraste simultane des Couleurs, Strassburg, 1839 ; new ed. Pa.ris, 1890. 

 3 Joh. Miiller, Handb. d. Physiol, n. 1837 ; H. Meyer, Ann. d. Physik. xcv. 170, 1855. 

 « Arch. J. d. rjes. Phy.^iol. xli. 1, 1887. * Ibid. xli. 358, 1887. 



« Ibid. XLii. 119, 1888. ' 1st ed. p. 406; 2nd ed. p. 559. 



* Arch.f. d. ges. Physiol, xlvii. 236, 1890 ^ Sitz. d. Wiener Akad. 1872-4. 



" Philos. Stud. IV. 112, 1887. " Physiol, d. Netzhaut, 1865. 



