132 RECENT PROGRESS IN OPTICS. 



much important work in this domain. The selection of hues for the 

 three primary color sensations has been slightly modified. Young 

 selected the two extremes of the spectrum, red and violet, together 

 with green, which is about midway between them. The hues now 

 accepted by Helmholtz and those who follow his lead, including the 

 great majority of physicists, are a highly saturated carmine red, an 

 equally saturated ultramarine blue, and a yellowish green, correspond- 

 ing somewhat to that of vegetation. The red and blue agree with 

 those previously determined by Hering, but the rivalry between the 

 two schools on the subject of color sensation continues, and perhaps 

 will last through a period commensurate with the difficulty of devising 

 crucial experiments. 



Independent theories of color sensation have been brought out by 

 Mrs. Franklin (Christine Ladd Franklin, a Eine neue Theorie der 

 Lichtempfundungen," Zeitschrift fur Psychologie und Physiologie der 

 Sinnesorgane, 1892), in America, and by Ebbinghaus ( u Theorie des 

 Farbenseheus," ibid., 1893), in Germany. The former particularly is 

 worthy of much more extended notice than can here be given. It may 

 perhaps be quite properly called a chemical theory of vision. Light is 

 always bringing about chemical changes in external objects, and the 

 eye is the one organ whose exercise requires the action of light, while 

 such chemical action is implied in the performance of most of the 

 bodily functions, such as the assimilation of food and the oxidation of 

 the blood. The bleaching action of light upon the visual purple, which 

 is continually formed on the retina, has been known ever since the 

 discovery of this in 1877 by Kiihne, who secured evanescent retinal 

 photographs in the eyes of rabbits. Mrs. Franklin considers that light 

 sensation is the outcome of photochemical dissociation of two kinds of 

 retinal molecules that she denominates gray molecules and color mole- 

 cules, of which the latter arise from the gray molecules by differentia- 

 tion in such a way that the atoms of the outer layer group themselves 

 differently in three directions, and the corresponding action^of light 

 of proper wave length gives rise to the three fundamental color sensa- 

 tions. She develops the theory with much skill, applying it particu- 

 larly to the phenomena of retinal fatigue and color blindness. To the 

 objection that there is no direct proof of the existence of the assumed 

 gray and color molecules, it may be answered that Helmholtz himself 

 fully recognized the uncertainty of the assumption that three different 

 sets of nerves respond to the three fundamental color sensations, and 

 he admitted that these may be only different activities in the same 

 retinal cone. The supposition of three adjacent cones, responding 

 respectively to the three fundamental sensations, is made only for the 

 sake of greater convenience in discussion. 



Indeed, there is still much for us to learn regarding the nature of 

 color sensation. Among the yet unexplained phenomena are those of 

 simultaneous color contrast. The fact that a small brightly colored 



