732 REPORT — 1892. 



•waves in the visible and invisible spectruai is frequency of undulation, or, otherwise 

 expressed, a diflerence in wave-length. The chromatic — that is, the colour-produc- 

 ing — effects of the ether-waves depend on tlieir power of exciting sensations of 

 colour, which vary with their frequency of undulation. 



Although the retina is extremely sensitive to differences in the frequency of 

 ether-waves, it is not equally so for all parts of the spectrum. In the red and blue 

 portions, the frequency varies considerably without producing marked difference 

 of colour effect, but in the region of yellow and green, comparatively slight varia- 

 tions in frequency produce appreciable differences of colour sensation. One striking 

 difference between the effect of ether-waves on the eye and sound-waves on the ear 

 is the absence of anything corresponding to the octave of tone sensation. The 

 ether-waves in the ultra-violet, which have twice the frequency of those of the red 

 end of the spectrum, give rise to no sense of redness, but merely that of a bluish- 

 grey. Even within the octave there are no harmonies or discords of colour sense 

 corresponding to those of tone sensation. 



Colours are commonly defined by three qualities or constants — hue, purity, and 

 brightness. Their hue depends upon the chromatic eff"ect of frequency of undulation 

 or wave length. Their purity or saturation depends on freedom from admixture 

 with sensations produced by other colours or by white light. Their brightness or 

 luminosity depends on the degree to which the sensory mechanism is stimulated. 

 The loudness of sound depends on the amount of excitement produced in the 

 auditory mechanism by the amplitude of sound waves ; but a sound with small 

 amplitude of undulation may seem loud when the nerve apparatus is unduly 

 sensitive. The brightest colour of the spectrum is orange-yellow, but it does not 

 follow that the amplitude or energy of the ether-waves is greater than in the 

 region of dull red. There is no physical evidence of greater amplitude in the 

 orange-yellow, and its greater luminosity is no doubt purely subjective, and arises 

 from the greater commotion induced in the sensory mechanism. 



The theory of colour sense long ago proposed by Sir Isaac Newton ' is now 

 commonly treated with what seems to me very undeserved neglect. Newton 

 supposed that the rays of light induce vibrations in the retina which are trans- 

 mitted by its nerve to the sensorium, and there induce different colour sensations 

 according to the length of the incoming vibrations — the longest producing sensa- 

 tions of red and yellow, the shortest blue and violet, those of medium length a 

 sense of green, and a mixture of them all giving a sense of whiteness. At the 

 beginning of this century Thomas Young proposed a theory which seems to have 

 been intended as a modification of that suggested by Newton rather than as a 

 substitute for it. Young supposed that the ether-waves induce vibrations in the 

 retina ' whose frequency must depend on the constitution of its substance ; but as 

 it is almost impossible to conceive that each sensitive point of the retina contains 

 an infinite number of particles, each capable of vibrating in unison with every 

 possible undulation, it becomes necessary to suppose the number limited to three 

 primary colours, red, yellow, and blue, and that each sensitive filament of the 

 nerve may consist of three portions, one for each principal colour.' ^ Soon after- 

 wards he substituted green for yellow, and violet for blue, so that he came to 

 regard red, green, and violet as the three fundamental colour sensations, by mixtui-e 

 of which in varying proportions all other colours, including white, are produced. 

 Young believed that his suggestion ' simplified the theory of colours, and might 

 therefore be adopted with advantage until found inconsistent with any of the 

 phenomena.' 



Young's trichromic theory of colour sense was adopted by Clerk-Maxwell, and 

 Von Helmholtz amplified the ' three portions' supposed by Young to exist in 'each 

 sensitive filament of the nerve ' into three distinct terminals, each having its 

 own nerve fibre. He supposed that each terminal contains a diflferent visual 

 substance capable of being decomposed by light ; that when the substance in 

 the red nerve terminal undergoes chemical change its nerve fibre is stimulated, 



' See quotations from Newton hy Thomas Young, ' On the Theory of Light and 

 Colours,' Phil. Trans. Lond., 1802. 

 •■' Ibid., p. 12. 



