A Manual Of Photography

296 PRACTICE OF PHOTOGRAPHY. experiment to be entirely dependent on the influence of the red rays, or that portion of the sunbeam which appears to have the greatest heating power : hence regarded as the seat of greatest calorific power. I have before mentioned the peculiar state of equilibrium in which the paper is when wetted with the hydriodate, and that a slight difference in the incident light will either bleach or blacken the same sheet. If four glasses, or coloured fluids, be prepared, which admit respectively the blue, green, yellow and red rays, and we place them over an hydriodated paper, having an engraving superposed, it will be bleached under the influence of the blue light, and a perfect picture produced ; while, under the rays transmitted by the green glass, the drawing will be a negative one, the paper having assumed, in the parts whicli re- present the lights, a very defined blackness. The yellow light, if pure, will produce the same effect, and the red light not only induces a like change, but occasions the dark parts of the en- graving to be represented in strong lights : this last peculiarity is dependent on the heating rays, and opens a wide field for in- quiry. My point now, however, is only to show that the darken- ing of the finished photograph is occasioned by the least refrangible rays of light ; whereas its preparation is effected by the most refrangible. I know not of any other process which shows, in a way at once so decided and beautiful, the wonderful constitution of every sunbeam which reaches us. Yet this is but one of numerous results of an analogous character, produced by these opposite powers, necessary to the constitution of that solar beam, which is poured over the earth, and effects those various changes which give to it diversified beauty, and renders it conducive to the well-being of animated creatures. Before quiting this branch of the art, it will be interesting to examine the modifications which have been introduced by some continental inquirers. M. Lassaigne, who has claimed priority in the use of the iodide of potassium, saturated his paper with a sub-chloride of silver, which was allowed to assume a violet-brown colour, and it was then impregnated with the iodidated solution. M. Bayard simply allowed ordinary letter paper, prepared according to Mr. Talbot's method, to blacken by light. He then steeped it for some seconds in a solution of iodide of potassium, and laying it on a slate he placed it in the camera. M. Verignon introduced a somewhat more complicated process. His directions are,—White paper should first be washed witli water acidulated by hydrochloric (muriatic) acid;