The destruction by oxidizing agents of the effect produced by light was then experimentally demonstrated, and photographs of the spectrum showed that this effect was increased by the action of light itself. Thus, when immersing a plate sensitive to all radiations, visible and invisible, in a very dilute solution of nitric acid, bichromate of potash, or hydroxyl, it was shown that if the plate were exposed to light, first the parts acted upon by the red rays were reduced before the parts not acted upon at all by the spectrum, thus conclusively proving that light itself helped forward the oxidation or so-called solarization of the image. It thus became a struggle, under ordinary circumstances, between the reducing action on the normal salt and the oxidizing action on the altered salt as to which should gain the mastery. If the reducing action of any particular ray were the most active, then a negative image resulted, whereas if the oxidizing action were in the ascendant, a positive image resulted. Thus, in determining the action of light on a particular salt, this antagonism had to be taken into account, and exposure made with such precautions that no oxidizing action could occur, as would be the case if an inorganic sensitizer, such as sulphite of soda, were used. The reversal of the image by soluble haloid salts, such as bromide of potassium, was then dwelt upon with experimental demonstration. It was shown that the merest trace of soluble haloid would reverse an image by the extraction of bromine from it, and the fact that the most refrangible part of the spectrum was principally efficacious in completing this action showed how necessary it was to avoid falling into error when analyzing photographic action by the spectroscope. A reference was next made to gelatine plates, in which, owing to their preparation, reversal through the above cause was most likely to take place, and a plate soaked in sulphite of soda and exposed in the camera for a couple of minutes--a time largely in excess of that necessary to give a reversal under ordinary circumstances--proved the efficacy of the oxygen absorber, the image remaining in its normal condition after development. The lecturer closed his remarks by showing the different molecular states of iodide, bromide, and chloride of silver, as produced by different modes of preparation. The color of the film by transmitted light in every case indicated the effect which was likely to be produced on them, and the photographed spectrum in each of them showed the remarkable differences that were found. The points raised by Captain Abney at different times are well worthy the study of scientific photographers, since strict attention to the modes of exposure to the spectrum, to the instruments employed, and to the source of light used can alone insure accuracy in comparative experiments.--_Br. Jour. of Photo_. * * * * * SALT AND LIME.