198 COLOUR VISION 



bleaching of visual purple with higher intensities, as the basis of the blue 

 sensation, which is therefore also carried out by the rods and is absent 

 from the fovea. 



V. Kries^, also independently of any knowledge of Schultze's paper 

 and apparently of Parinaud's writings, elaborated a similar theory, the 

 duplicity theory (DupUzitdtstheorie), which has received wide accepta- 

 tion. It will be considered in Section II. 



The physical basis of colour vision, as exemplified in the wonderful 

 researches of Sir Isaac Newton, opened up the ground for the modern 

 theories. Newton himself compared the simple colour notes of the 

 spectral scale to the tones of the musical scale. This idea was elaborated 

 by Hartley, Young, Drobisch, and others, but was soon found wanting^. 

 Attempts to explain the facts of colour-mixtures directly from the 

 undulatory theory of light by Challis, Grailich, and later Charpentier 

 have also proved inadequate^. 



More recently Hartridge^ has attempted to give a purely physical 

 explanation of the sensation of yellow. Yellow (650 [xfu. — 560 [jl[jl) is 

 accurately matched by a suitable mixture of red (740 /x/x — 690 /^/a) and 

 green (550 ju./x— 510 yu/x) if a small quantity of white light is added to 

 the yellow. By drawing on mathematical paper two sine curves of 

 equal amphtude, which have periods corresponding approximately to 

 the mean wave-lengths of red and green components of this compound 

 yellow, it will be found that their summation gives a curve which 

 approximates closely to a sine curve of wave-length 620 /x/x. It shows 

 beats at every 2000 ^/x ; at the nodes the amplitude is almost zero 

 whereas at the antinodes it is nearly double that of either of the curves 

 of which it is compoimded. By compounding a red and green wave 

 motion one has obtained a yellow wave motion, the mean wave-length 

 of which approximates closely with that found by experiment. 



If this hypothesis were accurate the combination of the same red and 

 green, polarised at right angles to each other, should fail to arouse the 

 sensation of yellow ; but such is not the case. Further, although the 

 compound curve obtained by combining the sine curves corresponding 

 to the red and green has between any two consecutive minima a period 

 equal to that of the yellow light, yet at each minimum the curve has 



^ Especiallj' in Bericht d. naturf. Ges. zu Freiburg i. B. ix. 2, 1894 ; Arch. f. Ophth. 

 XLn. 3, 95, 1896 ; Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. ix. 81, 1896. 

 2 V. Helmholtz, 3rd ed. n. 97. 



' Ibid. n. 129 ; Charpentier, La Lmniere et les Couleurs, Chap, xii, Paris, 1888. 

 * Proc. of the Physiol. Soc, J. of Physiol. XLV. p. xxix, 1913. 



