70 COLOUR VISION 



be made for macular pigmentation (vide infra, p. 124). The deduction 

 is therefore more accurate if paracentral and peripheral regions are 

 compared 1 . Care must be taken that light adaptation is complete and 

 has been induced by colourless light, and that long exposure to coloured 

 lights is avoided. 



Ole Bull 2 was the first to employ stimuli the saturations and lumi- 

 nosities of which were equalised and to discover colours which do not 

 change in hue in peripheral vision. His ' physiologically pure " 

 colours were a purplish red, a bluish green, a yellow, and a blue. 

 He found that the limits of the purple-red zone coincide with those 

 of the blue-green zone, and that those of the yellow and blue approxi- 

 mately coincide. The yellow-blue zone is considerably wider than 

 the red-green zone. He employed colour papers. 



Hess 3 , also using coloured papers, similarly showed that with light 

 adaptation colour pairs of fixed size, intensity, and saturation can be 

 selected which lose their colour simultaneously and form a grey match 

 when viewed peripherally, viz. green (495 /zju.) and red mixed with a small 

 amount of blue; and yellow (574*5 fi/ju) and blue (471 yufi). These 

 colours are complementary. All colours of greater wave-length than 

 549 fjLjji approximate yellow, all of less wave-length blue, in passing 

 from central to peripheral vision. Thus orange and green-yellow 

 become yellow ; blue-green, violet and purple become blue. Hence 

 there are only four colours which gradually become paler without 

 altering their colour tone, ultimately becoming colourless, as one passes 

 from central to peripheral vision. Hess calls these colours " invariable " 

 red, yellow, green and blue. The limits of the zones at w T hich the 

 colours become colourless vary with conditions already mentioned, 

 but the limits for invariable red and green are the same, as also for 

 invariable yellow and blue. 



Hegg 4 , Baird, and Dreher 5 confirmed Hess's results. Baird found 

 the '' stable ' : or invariable colours to be a purplish-red, a yellow 

 (about 570 /z/z), a bluish-green (about 490 ^}, and a blue (about 460 /A/.I). 

 Dreher's values for the three last were 568 /x/x, 483 /u/z, and 461 /JL/JL. 

 Baird found that the coincidence of the zones of each pair of stable 

 colours holds good for the dark-adapted as well as for the light-adapted 



1 Hess, Arch. f. Ophth. xxxv. 4, 1, 1889 ; v Kries, Ztsch. f. Psyclwl. u. Pin/si ol. d. 

 Hinnwory. xv. 263, 1897. 



2 Arch. /. Ophth. xxvn. 1, 54, 1881 ; xxix. 3. 71, 1883. 

 :1 Ihirl. xxxv. 4, 1, 1889. 



4 Ibid, xxxvm. 3, 145. 1892; Ann. d'ocul. fix. 321, 1893; rxi. 122, 1894. 



5 Zlncli.f. Sinneaphysiol. xr,vi. 1, 1911. 



