A Manual Of Photography

ON THE THEORY OF THE DAGUERREOTYPE. 145 been taken, and resort to the tedious process of cleaning and preparing another ; it is only necessary to treat the plate in the manner already pointed out, and it is again equal in every respect to a newly-prepared plate ; and this treatment may be repeated, until, by the slow accumulation of too thick a film of iodide of silver, the plate no longer possesses the same degree of sensitiveness to light." The researches of M. Claudet are of considerable importance, particularly as the observation of a thoroughly practical photo- graphic artist. The phenomena which M. Claudet considers have not yet been satisfactorily explained, and of which he treats, are those referring to the following points:— 1. "What is the action of light on the sensitive coating? 2. How does the mercurial vapour produce the daguerreotype image ? 3. Which are the particular rays of light that impart to the chemical surface the affinity for mercury? 4. What is the cause of the difference in achromatic lenses between the visual and photogenic lenses? why do they con- stantly vary ? 5. What are the means of measuring the photogenic rays, and of finding the true focus at which they produce the image? At the meeting of the British Association at Swansea, M. Claudet expressed his opinion that the decomposition of the chemical surface of the daguerreotype plate, by the action of certain rays of light, produced on that surface a white precipi- tate, insoluble in the hyposulphite of soda, which, when examined by the microscope, had the appearance of crystals reflecting light, and which, when seen by the naked eye, were the cause of a positive daguerreotype image. These were probably particles of pure white silver. Tire opinion of Daguerre himself, and other writers, was, that the action of light on the iodide of silver had only the effect of darkening the surface, and consequently of producing a negative imao-e. But it escaped them, that, under the darkened iodide » of silver, another action could take place after a continued ex- posure to light, and that the hyposulphite of soda washing could disclose a positive image. M. Claudet proved this fact in ob - taining, by the action of light only, and without mercury, images having the same appearance as those developed under the action of mercurial vapour. This direct and immediate effect of light is certainly remarkable ; but the daguerreotype process is not founded on that principle, on account of the slowness of its action; and it is fortunate that, long before light can produce L