TRANSACTIONS OF SECTION D. 733 



and the excitement travels to a cell in the brain by whose specific activity the 

 sensation of red arises. In like manner, when the visual substances in the green 

 and violet terminals are decomposed, nerve impulses travel through different 

 fibres to different cells in the vision centre, by whose specific activities the 

 sensations of green and violet arise. "With Ilelmholtz there was no question 

 as to difference in quality of sensation depending on difference in frequency of 

 nerve vibration arriving in the sensorium ; no such hypothesis was entertained by 

 him either for tone or for colour sensation. "With sight, as with hearing, he 

 supposed that the function of frequency of undulation virtually stops at the nerve 

 terminals in the eye and ear, and that the frequency of undulation of the physical 

 agent lias no correlative in the quality of motion passing from the receiving ter- 

 minal to the sensory cell. He believes that the different Irequencies of ether-waves 

 simply excite chemical changes in ditf'erent nerve terminals. He expressly states ' 

 that the molecular commotion in the nerve fibres for red, green, and violet is 

 identical in kind, and that its different effects depend on the specific activities of 

 the different cells to which it passes in the sensorium. It is evident that Helmholtz 

 entirely dismissed the Newtonian theory of the production of different qualities of 

 colour sense, and substituted for i t the doctrine of his own great teacher, Johannes 

 Miiller. 



The theory of Young and Helmholtz offers an explanation of so many facts, 

 and has at the same time provoked so much criticism, that I must enter more 

 fully into some of its details. On this theory, the sense of white or grey is sup- 

 posed to result from a simultaneous and duly balanced stimulation of the red, green, 

 and violet terminals. The red terminals are supposed to be excited chiefly by the 

 longer waves in the region of the red, orange, and yellow, but also by the shorter 

 undulations extending as far as Fraunbofer's line F at the beginning of the blue. 

 In like manner, the green terminals are excited cliiefly by the waves of medium 

 length, and to a less extent by the waves extending to about in the red, and by 

 the shorter waves extending to G in the violet. The violet terminals are stimulated 

 most powerfully by the shorter undulations between F and G, but also by the 

 longer ones reaching as fiir as D in the yellow ; therefore, optically homogeneous 

 light from any part of the spectrum, e.xcept its extreme ends, does not usually 

 give rise to a pure colour sensation ; all three primary sensations are present, and 

 consequently the colour inclines towards white — the more, the stronger the light. 



The experimental facts in support of Young's theory are familiar to all who 

 have studied physics. Compound colour sensations may be produced by causing 

 light of different wave lengths to fall simultaneously or in rapid succession on the 

 same part of the retina. The commonest experimental device is to rapidly whirl 

 discs with sectors of difl'erent colours, and observe the results of the mixed sen- 

 sations ; or to cause the images of coloured wafers or papers to fall simultaneously 

 on the retina by Lambert's method ; or to transmit light through glass of different 

 colours, and cause the different rays to fall on the same surface ; or to mix pure 

 homogeneous light from different parts of the spectrum. For obvious reasons, the 

 last method yields the mo.«t trustworthy results. We cannot, by any mixture of 

 homogeneous light from different parts of the spectrum, obtain a pure red or green 

 sensation, and, according to Ilelmholtz, the same holds true of violet. On the 

 other hand, a mixture of homogeneous rays from the red and green parts produces 

 orange or yellow, according to the proportions employed. A mixture of rays 

 from the green and violet gives rise to intermediate tints of blue, and a mixture 

 of red and violet light produces purple. Therefore, Young regarded red, green, 

 .'ind violet as primary sensations, and orange, yellow, and blue — ^just as much as 

 •jiurple — he regarded as secondary or compound sensations. Grassmann discovered 

 that to obtain r. sense of white or grey it is not necessary to mingle rays from the 

 red, green, and violet portions of the spectrum. He found that he could obtain a 

 white sensation bj' mixing onlj^ t^^•o optically homogeneous rays from several parts 

 <)f the right and left sides of the spectrum. The pairs of spectral colours which 

 he found complementary to each other are red and greenish-blue, orange and 



' Von Helmholtz, Uandhnch dcr phj.nologischen Optih, 2nd edition, 1892, p. 350. 



