776 



YOUNU-HELMHOLTZ THEORY OF COLOUR SENSATION. 



By means of the colour table we may ascertain the mixed colour of three or more colours. 

 For example, it is required to find the mixed colour resulting from the union of the point, a 

 (pale yellow), * (fairly saturated bluish -green), and c (fairly saturated blue). On the three 

 points place weights corresponding to their intensities, and ascertain the centre of gravity of the 

 weight, a, b, c ; it will lie at p. It is obvious, however, that the impression of this mixed 

 colour, whitish green-blue, can be produced by green-blue + white, so that p may be also the 

 centre of gravity of two weights, which lie in the line connecting white and green-blue. 



We may describe a triangle, V, Gr, R, about the colour table so as to enclose it completely. 

 The three" fundamental or primary colours lie in the angles of this triangle, red, green, violet. 

 It is evident that each of the coloured impressions, i.e., any point of the colour table, may be 

 determined by placing weights corresponding to the intensity of the primary colours at the 

 angles of the triangle, so that the point of the colour table, or what is the same thing, the 

 desired mixed colour, is the centre of gravity of the triangle with its angles weighted as above. 

 The intensity of the three primary colours, in order to produce the mixed colour, must be re- 

 presented in the same proportion as the weights. 



Theories. Various theories have been proposed to account for colour sensation. 



1. According to one theory, colour sensation is produced by one kind of element present in 

 the retina, being excited in different ways by light of different colours (oscillations of the light- 

 ether of different wave-lengths, number of vibrations, and refractive indices). 



2. Young-Helmholtz Theory. The theory of Thomas Young (1807) and v. 

 Helmholtz (1852) assumes that three different kinds of nerve-elements, correspond- 

 ing to the three primary colours, are present in the retina. Stimulation of the 

 first kind causes the sensation of red, of the second green, and of the third violet. 



The elements sensitive to red are most strongly excited by light with the longest wave- 

 length, the red rays ; those for green by medium wave-lengths, green rays ; those for violet by 

 the rays of shortest wave-length, violet rays. Further, it is assumed, in order to explain a 

 number of phenomena, that every colour of the spectrum excites all the kinds of fibres, some of 



Gr 



Fig. 562. 

 than feebly, otlicrs strongly. Suppose in fig. 562 the colours of the spectrum are arranged in their 

 natural order from red to violet horizontally, then the three curves raised upon the abscissa 

 might indicate the strength of the stimulation of the three kinds of retinal elements. The 

 continuous curve corresponds to the rays producing the sensation of red, the dotted line that 

 of green, and the broken line that of violet. Pure red light, as indicated by the height of the 

 ordinates in R, strongly excites the elements sensitive to red, and feebly the other two kinds 

 of terminations, resulting in the sensation of red. Simple yellow excites moderately the 

 elements for red and green, and feebly those for violet = sensation of yellow. Simple green 

 excites strongly the elements for green, but much more feebly the two other kinds = sensation 

 of green. Simple blue excites to a moderate extent the elements for green and violet; more 

 feebly those for red = sensation of blue. Simple violet excites strongly the corresponding 

 elements, feebly the others = sensation of violet. Stimulation of any two elements excites the 

 impression of a mixed colour ; while, if all of them be excited in a nearly equal degree, the 

 sensation of white is produced. As a matter of fact, the Young-Helmholtz theory gives a 

 simple explanation of the phenomena of the physiological doctrine of colour. It has been 

 attempted to make the results obtained by examination of the structure of the retina accord 

 with this view. According to Max Schultze, the cones alone are end-organs connected with 

 the perception of colour. The presence of longitudinal striation in their outer segments is 

 regarded as constituting them multiple terminal end-organs. Our power of colour sensation, 

 s) far as it depends on the retina, would, on this view of the matter, bear a relation to the 

 number of cones. The degree of colour sensation is most developed in the macula lutea, which 

 contains only cones, and diminishes as the distance from the point increases, while it is absent 

 m the peripheral parts of the retina. The rods of the retina are said to be concerned only with 

 the capacity to distinguish between quantitative sensations of light. 



3. Henng'g Theory. Ew. Hering, in order to explain the sensation of light, proceeds from 

 the axiom stated under 1, p. 775. What we are conscious of, and call a visual sensation, is 

 the physical expression for the metabolism in the visual substance ( ' Schsubstanz "), i.e., 

 in those nerve-masses which are excited in the process of vision. Like every other corporeal 



