PHOTOGENIC DRAWING, the name given by Mr. H. F. Talbot, the inventor ur discoverer of it, to a "new art," which, though not identical, is very similar to that of :M. Daguerre. The outline of the process is as follows : A piece of copper is plated in the usual way with silver by passing the metals together through a rolling mill, and is then cut into pieces of a proper size. The silver surface is carefully polished, and cleansed by wiping it over with a piece of cotton dipped in dilute nitric acid, washing, and drying. When thus duly prepared—and much depends upon the manner in which these preliminary operations are performed and the materials used—the plate is subjected to the diffused vapor of iodine, which forms a slightly brown or yellow film upon the silver ; it is then ready to be subjected to the action of the image to be represented, which is thrown upon it, care being taken to exclude all other light, by an instrument upon the principle of the camera obscure.. In the course of a few seconds or minutes, the requisite time depending upon the intensity of the light, the plate is removed; and though nothing is as yet visible upon it it has re ceived the image, which is brought out and rendered evident by subjecting it, inclined at an angle of about to the vapor of mercury. This operation is per
formed in a box with a glass side, at the bottom of which is a basin oi mercury, heated to about 170°, so that the operator may see the progress of the appearance of the image, and remove the plate when it is perfect ; but light must be as far as possible excluded, and more especially daylight. The plate is then washed by cautious immersion in a solution uf hy po-sulphite of soda, and lastly with boil ing distilled water, and allowed to dry : it is now perfect, may he exposed to light without injury ; but must be carefully protected from all friction by covering it with a glass. The action of the various shades of light upon the film of iodine, and the subsequent influence of the mer curial vapor upon which the visibility of the picture depends, have not been satis factorily explained, and require further experimental elucidation. The perfection of the drawing, and the extraordinary manner in which the minutest details are represented, we have noticed in our for mer article ; they must, however, be seen to be accurately judged of and duly appreciated.