384 W. LeConte Stevens—Lecent Progress in Optics. 
Physiological Optics. 
The temptation to dilate upon recent progress in physiologi- 
cal optics has to be resisted. The revision of Helmholtz’s 
great book on this subject was interrupted by the death of the 
distinguished author, but the last part is now approaching 
completion under the care of his pupil, Arthur Konig, who in 
conjunction with Diederici has done 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 satu- 
rated carmine red, an equally saturated ultramarine blue, and. 
a yellowish green, corresponding 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 difficuity of devising crucial 
experiments. 
Independent theories of color-sensation have been brought 
out by Mrs. Franklin* in America and by Ebbinghaus} 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. ‘lhe 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 lght-sensation 
is the outcome of photo-chemical dissociation of two kinds of 
retinal molecules that she denominates gray molecules and color 
molecules, of which the latter arise from the gray molecules by 
differentiation in such a way that the atoms of the outer layer 
group themselves differently in three directions, and the corre- 
sponding action of light of proper wave length gives rise to the 
three fundamental color-sensations. She develops the theory 
with much skill, applying it particularly to the phenomena of 
retinal fatigue and color-blindness. To the objection that 
* Christine Ladd Franklin, ‘‘ Hine neue Theorie der Lichtempfindungen,” Zeit- 
schrift fiir Psychologie und Physiologie der Sinnesorgane, 1892. 
+ H. Ebbinghaus, ‘‘ Theorie des Farbensehens,”’ 1. ¢., 1893. 
