PHOTOGRAPHY. *, 



or, as it is now called, a negative. Such a blackened paper 

 with the white image on it could be used to cover a second 

 piece of sensitive paper, which on exposure to light would 

 show the lace as black on a white ground ; and this he 

 termed a positive picture. Xote the advantage of this process 

 over the Daguerrean, which for each copy of an object 

 required a fresh exposure in the camera. 



Hitherto we have dealt exclusively with the method of 



Negative. 



FIG. l. 



producing silver iodide by the direct contact of the metal 

 with the halogen ; but the same results can be obtained 

 by chemical decomposition. Silver iodide may be precipi- 

 tated by mixing a solution of potassium iodide with silver 

 nitrate. 1 This was the method adopted by Fox Talbot in 

 the calotype process, patented in 1841. He added sufficient 

 potassium iodide to a solution of the silver nitrate to preci- 

 pitate silver iodide, and then an excess to redissolve it. Such 

 a solution he brushed over a piece of paper, which when dry 

 he washed, leaving on it primrose-coloured silver iodide. In 

 this state the silver compound was insensitive to light, as 

 there was nothing present (besides the paper) to take up 

 iodine. We have here a piece of paper prepared as in- 

 dicated, and it is now exposed to the strong glare of the 

 * KI + AgN0 3 = Agl + KNO,,. 



