SECT. XXIV. M. DAGUERRE. 205 



by this simple process, and a direct picture may be produced at 

 once by using photographic paper already made brown by ex- 

 posure to light. 



While Mr. Fox Talbot was engaged in these very elegant 

 discoveries in England, M. Daguerre had brought to perfection 

 and made public that admirable process by which he has com- 

 pelled Nature permanently to engrave her own works ; and thus 

 the talents of France and England have been combined in bring- 

 ing to perfection this useful art. Copper, plated with silver, 

 was 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 carefully from light, it must be placed in the 

 focus of a camera obscura, where an invisible image of external 

 objects 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 proportion to the 

 degree of illumination, the shading of the picture is perfect. 

 The image is fixed, first by removing the iodine from the plate 

 by plunging it into hyposulphite of soda, and then washing it 

 in distilled water ; by this process the yellow colour is destroyed, 

 and in order to render the mercury permanent, the plate must 

 be exposed a few minutes to nitric vapour, then placed in nitric 

 acid containing copper or silver in solution at a temperature of 

 61i of Fahrenheit for a short time, and lastly polished with 

 chalk. This final part of the process is due to Dr. Berre, of 

 Vienna. 



Nothing can be more beautiful than the shading of these chiar- 

 oscuro pictures when objects are at rest, but the least motion 

 destroys the effect ; the method therefore is more applicable to 

 buildings than landscape. Colour is wanting ; but the researches 

 of Sir John Herschel give reason to believe that even this will 

 ultimately be attained. 



The most perfect impressions of seaweeds, leaves of plants 



