Page:Popular Science Monthly Volume 45.djvu/565

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ferent simple or complex colors. This is what is called isochromatism.

By the aid of these improvements M. Lippmann has succeeded in fixing on his plates images of marvelous beauty. The colors have an inconceivable brightness and delicacy of shading. They have nothing in common with painted copies of photographs, which simply enhance the photographic images with coloring. The photographic proofs obtained by M. Lippmann have a strength of coloring and a richness of tone which no water-color picture has ever attained. This is because, in his photography, the registration of the color is combined with the accumulation of all the colored rays.

It is not necessary to say that the learned professor in the Sorbonne has not sought to draw an industrial profit from his invention. It is free to all who may hereafter wish to direct their investigations that way. There remains much still to be done before all the improvements can be given to science. The problem now is to advance from the fixation of the colors on the sensitive plates to their reproduction on paper. Theory permits the prediction that regular reflection by a metallic mirror may be replaced before long by the diffusion of light over a dead surface. It is, then, permissible to hope, without contradiction of the theory of interferences, that the multiplication of proofs by simple printing on paper is only a matter of time. It is easy to understand how much the arts and science are interested in the progress of the photography of colors.

While the pigmentary colors used by painters are made of substances which light may change in the long run, interference colors, which are produced by the vibratory movement alone, depend solely on the physical and mechanical conditions of the experiment, and are not subject to alteration by time. Photography of colors will permit the faithful reproduction of the pictures of the masters, and will also assure the reproduction of meteorological phenomena which may be of considerable importance in future studies of astronomical science.—Translated for The Popular Science Monthly from the Revue des Deux Mondes.

A practical course of instruction in psycho-physiology was given in University College, London, during the Easter term, by Dr. L. E. Hill. The plan of the course was to take the student methodically over the several senses, and familiarize him with the methods by which the new branch of science known as physiological psychology or psycho-physics determines the precise manner in which sensation varies, both quantitatively and qualitatively, with variations of the stimulus, of the particular portion of the sensitive surface stimulated, etc. The Athenæum acknowledges the backwardness of England as compared with the United States and Germany in the systematic laboratory instruction of students in this subject.