AN important paper on the theory of colour photography is contributed to No. 6 of Wiedemann's Annalen, by Herr Otto Wiener. The paper deals with the methods of attacking this problem which are based, not upon the photography of the different constituents of coloured light and their subsequent recognition—like Mr. Ives's heliochromy and similar processes—but upon the direct production of colour by the influence of light upon certain chemical substances. The most recent, and in a way the most successful of these methods is that due to Lippmann, and the question raised by Herr Wiener is whether the old processes invented by Becquerel, Seebeck, and Poitevin are based upon interference colours like Lippmann's, or upon “body colours,” i.e. colours produced by partial absorption of the incident light. That Lippmann's colours are due to interference may be very simply proved by breathing upon a plate with a photograph of the spectrum, when the colours quickly wander towards the violet end, this result being due to an increase in the distance between the nodal layers. This experiment cannot be applied to a spectrum photographed by Becquerel's method. But Herr Wiener succeeded, by a simple and ingenious contrivance, altering the path of the rays through the coloured film by placing a rectangular prism on the plate, with its hypothenuse surface in contact with the spectrum. This experiment had the startling result that that part of the spectrum covered by the prism appeared strongly displaced towards the red. Hence Zenker's theory of Becquerel's process, enunciated in 1868, which ascribed the colours to interference, is substantiated. Instead of Becquerel's homogeneous sheet of silver chloride containing subchloride, Seebeck used the powder, and Poitevin mounted the salt on paper. In these two processes the effect described is not observed. Hence these colours are body colours in these two cases. The production of these body colours is a very mysterious process, but the author hopes that here will eventually be found a satisfactory solution of the problem. To account for the production of these colours he advances a remarkable theory which has a well-known analogy in comparative physiology. Given a collection of compounds of silver chloride and subchloride of indefinite proportions, such as those which Mr. Carey Lea calls by the collective name of “photochloride,” we must suppose according to the modern kinetic theories that they are undergoing a rapid series of successive modifications. When the red combination happens to be exposed to red light, it reflects it without absorption, and will therefore no longer be affected or changed by it. Similarly for the other cases. This is another process of “adaptation.” The author describes some experiments which prove that this is the true explanation, and points out the importance of this view, not only for colour photography, but for the production of colours in the animal world.