S26 M. DAGUERRE. [SECT. xxiv. 



a solution of iodide of potassium. For these liquids the 

 liquid hyposulphites have been advantageously substituted, 

 which are the most efficacious in dissolving and removing the 

 unchanged salt, leaving the reduced silver on the paper. The 

 calotype picture is negative, that is, the lights and shadows 

 are the reverse of what they are in nature, and the right- 

 hand side in nature is the left in the picture ; but, if it be 

 placed with its face pressed against photographic paper, 

 between a board and a plate of glass, and exposed to the sun 

 a short time, a positive and direct picture as it is in nature 

 is formed : engravings may be exactly copied by this simple 

 process, and a direct picture may be produced at once by 

 using photographic paper already made brown by exposure 

 to light. 



While Mr. Fox Talbot was engaged in these very elegant 

 discoveries in England, M. Daguerre had brought to per- 

 fection and made public that admirable process by which he 

 has compelled Nature permanently to engrave her own works ; 

 and thus the talents of France and England have been com- 

 bined in bringing to perfection this useful art. Copper, plated 

 with silver, is successfully employed by M. Daguerre for 

 copying nature by the agency of light. The surface of the 

 plate is converted into an iodide of silver, by placing it 

 horizontally with its face downwards in a covered box, in the 

 bottom of which there is a small quantity of iodine which 

 evaporates spontaneously. In three or four minutes the 

 surface acquires a yellow tint, and then, screening it care- 

 fully from light, it must be placed in the focus of a 

 camera obscura, where an invisible image of external ob- 

 jects will be impressed on it in a few minutes. When 

 taken out, the plate must be exposed in another box to the 

 action of mercurial vapour, which attaches itself to those 

 parts of the plate which had been exposed to light, but does 

 not adhere to such parts as had been in shadow ; and, as the 

 quantity of mercury over the other parts is in exact propor- 

 tion to the degree of illumination, the shading of the picture 



