HISTORY OF PHOTOGRAPHY. 183 



quickl,v develop itself. Having- mixed together about one dram of 

 a saturated solution of p?'otos>/Iphate of iron and two or three drams 

 of mucilage of gum arahic^ pour a small quantity into a fiat dish. 

 Pass the prepared side of the paper taken from the camera rapidly 

 over this mixture, taking care to insure complete contact in every part. 

 If the paper has been sufficiently impressed, the picture will almost 

 immediately appear, and the further action of the iron must be 

 stopped by the application of a soft sponge and plenty of clear water. 

 Should the image not appear immediately, or be imperfect in its de- 

 tails, the iron solution may l)e allowed to remain upon it a short time; 

 but it must then be kept disturbed by rapidly but lightly brushing it 

 up, otherwise numerous black specks will form and destroy the photo- 

 graph. Great care should be taken that the iron solution does not 

 touch the back of the picture, which it will inevitably stain, and, the 

 picture being a negative one, be rendered useless as a copy. A slight 

 degree of heat will assist the development of the image where the time 

 of exposure has been too short. 



The picture should be carefully washed to take off any superficial 

 blackness, and may then be permanently fixed by being soaked in 

 water to which a small quantity of ammonia, or, better still, hypo- 

 sulphite of soda, has been added. The paper must again be well 

 soaked in clean water, to clear it from the soluble salts, and may then 

 be dried and pressed. 



Exact copies of prints, feathers, leaves, etc., may be taken on the 

 succinated paper by exposing them to the light in the copying frame, 

 until the margin of the prepared paper, which should be left uncov- 

 ered, begins to change color very slightly. If the object to be 

 copied is thick, the surface must be allowed to assume a darker tint, 

 or the light will not have penetrated to the paper. 



Positive copies of the camera negatives are procured in the same 

 manner as the copies of the prints, etc., just described. Instead, 

 however, of using the iron solution, the paper must be exposed to the 

 light, in the frame, a sufficient time to obtain perfect copies. The 

 progress of the picture may be observed by turning up the corner of 

 the paper, and, if not sufficiently done, replacing it exactly in the 

 same position. They should be fixed with hyposulphite, as before 

 directed. 



At the meeting of the British Association at York in IS-t-t I 

 showed by a series of photographs that the 'protoHulphate of iron was 

 most effective in developing any photographic images on whatever 

 argentiferous preparation they may have been received. Everj^ sub- 

 sequent result has shown that with proper care it is the most ener- 

 getic agent for developing with which Ave are acquainted. Tlio 

 difficulty of obtaining and of preserving the salt free of any peroxide, 



