THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 305 



materials oniploved l)y the niamifactiirers of ]:)orcelain; it should 

 also be flat, very thin, and MMiiitraiisparcnt ; if too thin, so that there 

 would 1)0 a chaiuv of breakinij;', it may be attached by means of cement 

 to a piece of irlass to give it strength. The substance of the plate 

 should be sliohtly porous, so as to enable it to imbibe and retain a 

 STifficient (juantity of the chemical soluticms employed. To i)repare 

 the ])late for use, it is first required to give it a coating of albumen, or 

 white of eggs, laid on very evenly, and then gently dried at a fire. 

 According as thi> i)late is more or less porous, it requires more or less 

 of the albuminous coating; it is best to employ a very close-grained 

 porcelain, which requires but little white of egg. The prepared 

 plate may be made sensitive to light in the same way in which a sheet 

 of paper is rendered sensitive; and we generally find the same 

 methods applicable for photographic pictures on paper applicable 

 to those on porcelain plates, and one of the processes employed by the 

 patentee is nearly the same as that patented by Mr. Talbot in 1841. 



The prepared plate is dipped into a solution of nitrate of silver 

 made by dissolving 25 grains of nitrate in 1 ounce of water; or the 

 solution is spread over the plate uniformly with a brush. The plate 

 is then dried, afterwards dipped into a solution of iodide of potas- 

 sium of the strength of about 25 grains of iodide to 1 ounce of water, 

 again dried, and the surface rubbed clean and smooth with cotton. 

 The plate is now of a pale-yellow color, owing to the formation on 

 the surface of iodide of silver. The plate, prepared as above di- 

 rected, may be kept in this state until required, when it is to be ren- 

 dered sensitive to light by washing it over with a solution of gallo- 

 nitrate of silver, then placed in the camera ; and the image obtained 

 is to be rendered visible and sufficiently strengthened by another 

 washing of the same liquid, aided by gentle warmth. The negative 

 picture thus obtained is fixed by washing it with water, then with 

 bromide of potassium, or, what is still better, hyposulphite of soda, 

 and again several times in water. The plate of porcelain being semi- 

 transparent, positive pictures can be obtained from the above-men- 

 tioned negative ones by copying them in a copying frame. 



The picture obtained on porcelain can be altered or modified in 

 appearance by the application of a strong heat, a process not appli- 

 cable to pictures taken on paper. AVith respect to this part of their 

 invention, the patentees claim : 



The obtaining by means of a camera or copying frame pliotograpliic images 

 or pictures upon slalis or plates of porcelain. 



The second i)art relates to the process which has been discovered 



and improved upon by Mr. ISIalone, who is associated with Mr. Fox 



■ Talbot in the patent. The patentees' improvement is a method of 



obtaining more complete fixation of photographic pictures on paper. 



SM 1904 20 



