LIGHT. 97 



must be immersed in a solution which has the property of 

 dissolving chloride of silver, but not metallic silver. Iodide 

 of potassium will effect this ; and the paper being washed and 

 dried will then preserve a permanent image of the depicted 

 objects. This was the first and simple process of Mr. Talbot ; 

 but it is defective as to the purposes aimed at, in many points. 

 First, it is not sufficiently sensitive, requiring a strong light 

 and a long time to produce an image ; secondly, the lights 

 and shadows are reversed ; and thirdly, the coarse structure 

 of the finest paper does not admit of the delicate traces of 

 objects being distinctly impressed. These defects have been 

 to a great extent remedied by a process subsequently disco- 

 vered by Mr. Talbot, and which bears his name, and which 

 has led to the collodion process, and others unnecessary to be 

 detailed here. 



The photographs of M. Daguerre, with which all are now 

 familiar, are produced by holding a plate of highly-polished 

 silver over iodine. A thin film of iodide of silver is thus 

 formed on the surface of the metal ; and when these iodized 

 plates are exposed in the camera, a chemical alteration takes 

 place. The portions of the plate on which the light has im- 

 pinged, part with some of the iodine, or are otherwise changed 

 for the theory is somewhat doubtful so as to be capable of 

 ready amalgamation. When, therefore, the plate is placed over 

 the vapour of heated mercury, the mercury attaches itself to 

 the portions affected by light, and gives them a white frosted 

 appearance ; the intermediate tints are less affected, and those 

 parts where no light has fallen, by retaining their original 

 polish, appear dark : the iodide of silver is then washed off by 

 hyposulphite of soda, which has the property of dissolving it, 

 and there remains a picture in which the lights arid shadows 

 are as in nature, and the molecular uniformity of the metallic 

 surface enables the most microscopic details to be depicted 

 with perfect accuracy. By using chloride of iodine, or bro- 

 mide of iodine, instead of iodine, the equilibrium of chemical 

 forces is rendered still more unstable, so that images may be 

 taken in an indefinitely short period a period practically 

 instantaneous. 



It would be foreign to the object of this essay to enter upon 



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