A text-book of physiology for medical students and physicians . the extreme violet * Helmholtz, Handbuch der physiologisehen Optik, second edition,1896, I, 344. 356 THE SPECIAL SENSES. rays act more or less on all of the substances, and the resulting redor violet sensation, is, therefore, mixed to some extent with white,—that is, is not entirely saturated. The theory, as stated by Helm-holtz, held strictly to the doctrine of specific nerve energy, in assumingthat each photochemical substance serves simply as a means for theexcitation of a nerve fiber, and that the quality of the sensationarous
A text-book of physiology for medical students and physicians . the extreme violet * Helmholtz, Handbuch der physiologisehen Optik, second edition,1896, I, 344. 356 THE SPECIAL SENSES. rays act more or less on all of the substances, and the resulting redor violet sensation, is, therefore, mixed to some extent with white,—that is, is not entirely saturated. The theory, as stated by Helm-holtz, held strictly to the doctrine of specific nerve energy, in assumingthat each photochemical substance serves simply as a means for theexcitation of a nerve fiber, and that the quality of the sensationaroused depends on the ending of this fiber in the brain. The phe-nomenon of negative after-images finds a simple explanation in termsof this theory. If we look fixedly at a green object, for example,the corresponding photochemical substance is chiefly acted upon, andif subsequently the same part of the retina is exposed to white light,the red and violet substances, having been previously less actedupon, now respond in greater proportions to the white light, and. Fig. 153.—Schema to illustrate the Young-Helmholtz theory of color vision.—(Helm-holtz.) The spectral colors are arranged in their natural order,—red to violet. The curvesrepresent the intensity of stimulation of the three color substances: 1, The red perceivingsubstance; 2, the green perceiving; 3, the violet perceiving. Verticals drawn at an*point of the spectrum indicate the relative amount of stimulation of the three substancesfor that wave length of the spectrum. the after-image takes a red-violet—that is, purple—color. Manyobjections have been raised to the Young-Helmholtz theory. Ithas been urged, for instance, that we are not conscious that whiteor yellow sensations are blends or compounded color sensations;we perceive in them none of the supposed component elements aswe do in such undoubted mixtures as the blue-greens or the theory explains poorly or not at all the fact that on the periphery
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