A Manual Of Photography

40 HISTORY OF PHOTOGRAPHY. dace the best effect is pretty easily guessed at. When the proper yellow colour is produced, the plate must be put into a frame, which fits the camera obscura, and the doors are instantly closed upon it, to prevent the access of light. The figures represent this frame, fig. 6, with the doors, B B, closed on the plate ; and fig. 7, with the doors opened by the half circles, A A. D D are stops by which the doors are fastened until the moment when the plate is required for use. The third operation should, if possible, immediately succeed the second : the longest interval between them should not exceed an hour, as the iodine and silver lose their requisite photogenic properties.1 It is necessary to observe, that the iodine onglet never to be touched with the fingers, as we are very liable to injure the plate by touching it with the hands thus stained. Third Operation.—The third operation is the fixing of the plate at the proper focal distance from the lens of the camera obscura, and placing the camera itself in the right position for taking the view we desire. Tig. 8 is a perpendicular section, lengthwise, of Daguerre's camera. A is a ground glass by which 8. the focus is adjusted; it is then removed, and the photographic plate substituted, as in c, fig. 9. B is a mirror for observing the effects of objects, and selecting the best points of view. It is inclined at an angle of 45°, by means of the support, L. To adjust the focus, the mirror is lowered, and the piece of ground glass, A, used. The focus is easily adjusted by sliding the box, D, out or in, as represented in the plate. When the focus is adjusted, it is retained in its place by means of the screw, II. The object glass, J, is achromatic and periscopic; its diameter is about one inch, and its focal distance rather more than four- 1 This is contrary to thp experience of the author of this volume ; and Dr. Draper, of New York, states that he has found the plates improve by keeping a few hours before they are used ; and M. Claudet states, that even after a day or two the sensibility of the plates is not impaired.