COLOR VISION.

The perfectly satisfactory physiologic explanation of color sensation has not vet been made, as is demonstrated by the fact that in nearly all the text-books it is found necessary to state the two leading hypotheses, those of Helmholtz and Hering, side by side, though widely differing in their methods of accounting for the phenomena. One may be more favored than the other, but each has its defects. The latest critic of the Helmholtz theory, Tschiriew, in the Archives de Physiologie, Normale et Pathologique for October, asserts that it is defective in that it does not respond satisfactorily to Johannes Mueller's laws of specific energies, in that it assumes each spectral ray of a determined wave length can at once excite all three elements of the retina in different degrees and thus to give rise to these different sensations. It also fails to account for that type of true color-blindness, consisting