298 THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 



access of light. The figures represent this frame, figure G, witli the 

 doors, B B, closed on the plate, and figure 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 betAveen them should not exceed an hour, as the 

 iodine and silver lose their requisite photogenic properties." It is 

 necessary to observe that the iodine ought never to be touched with 

 the fingers, as we are very liable to injure the plate by touching it 

 Avith 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 Ave desire. 

 Figure 8 is a perpendicular 

 section, lengtliAvise, of Da- 

 guerre's camera. A is a 

 ground glass by Avhich the 

 focus is adjusted. It is 

 then removed and the pho- 

 tographic plate substituted, 

 as in C (fig. D) . B is a mir- 

 ror for observing the effects of objects and selecting the best points 

 of vieAv. It is inclined at an angle of 45° by means of the support 

 L. To adjust the focus, the mirror is loAvered 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 screAv IT. The 

 object glass J is achromatic and periscopic. Its diameter is about 1 

 inch, and its focal distance rather more than 14 inches. M is a stop 



oThis is conti'ary to the expeiMence of the author of this volume, and Doctor 

 Draper, of New York, states that he has found the plates Improve by l^eei)ing 

 a few liours before they are used, and M. CMaudet states that even after a 

 day or two the sensibility of the plates is not impaired. 



