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IN 1878 Ewald & Kiuhne measured the time necessary for.solutions of visual purple to become decolourized completely when heated to various temperatures. They also investigated the effects of NaCl and of the absence of water on the thermal bleaching, and concluded that the reactions were very similar to the coagulation of albumen by heat. Since very little is known about the chemical nature of visual purple, it was decided to extend these observations by making some quantitative measurements of its thermal bleaching at various hydrogen-ion concentrations. Apart from the theoretical interest, it is of practical importance to know the amount of decomposition which may be expected during certain operations on the substance.The experiments were carried out over the whole range of acidities within which visual purple is not very rapidly destroyed at room temperatures. The velocity of decomposition was followed by the progressive loss of optical density with time, and in what follows the term " decomposition of visual purple" means the loss of the typical colour since our only sure guide to the presence of visual purple is its characteristic wavelength absorption curve.

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This paper records the measurement of the absorption curves of a number of new forms of visual purple which have been found among fishes. For reasons which will be given immediately it was hoped that we could correlate these results either with the habits of the species or with the microstructure of their retinae, but the attempt was not successful. We believe the results in themselves to be important because they show that light-sensitive substances of varying absorptions can be found in the retina and it may not be impossible to discover substances which form intermediate links in the perception of colour. The retinae of fishes have attracted considerable attention from histologists because of their large and easily studied visual cells and the extent of their photomechanical changes. The results reported here on the types, distribution, and behaviour of the visual cells in the species examined are not claimed to be either exhaustive or final. The histology was carried out under difficulties and it was only possible to examine a few specimens of each species. It was found, however, that where other workers have also reported on the same retina (as in the elasmobranchs, eel, trout, etc.) the observations were always in agreement.