IN 1874 Dr. Vogel communicated to the scientific public that he had been able to photograph the least refrangible rays of the solar spectrum by using what are known as the Uranium Dry Plates prepared by the Uranium Dry Plate Company. At the same time he experimented with other bromide plates, using dyes to give them distinctive tints. He then enunciated that the sens itiveness of the plates for the hitherto unphotographed portion of the spectrum was due to the colours employed, and apparently all his efforts have been diverted in elaborating this idea. Last summer I commenced a series of similar experiments to see whether the discovery could be made of practical use in photography; and have arrived at the conclusion, that the colouring matter gives this extended sensitiveness owing to the compound of silver formed and not to the colour itself, in fact, that the tint given to the film necessitates a very prolonged exposure. The additions of resins, nitro-glucose, and other similar compounds containing carbon to the bromized and bromoiodized collodion soon convinced me that it was the organic salts of silver to which we must look for sensitiveness in the yellow red and ultra-red rays of the spectrum. Nearly every resinous body seems to prolong the photographic spectrum towards the ultra-red, and one or two in particular, also when the film is least colourless when viewed by transmitted light, that then it is probably in the most impressionable condition. Another point which is worthy of notice is, that a film dried from moisture, taking, in fact, the form of a dry plate, is always most readily acted upon by the ultra-red end of the spectrum. Probably this is due to the absorptive qualities possessed by the silver nitrate solution, and not really from an increased sensitiveness of the compound salts when dry.