112 CORRELATION OF PHYSICAL FORCES. 



a strong light, the chloride will be decomposed in all those 

 parts of the paper where the light is not intercepted, and we 

 shall have, by the action of light, a white image of the leaf 

 on a purple ground. If similar paper be placed in the focus 

 of a lens in a camera-obscura, the objects there depicted will 

 decompose the chloride, just in the proportion in which they 

 are luminous ; and thus, as the most luminous parts of the im- 

 age will most darken the chloride, we shall have a picture of 

 the objects with reversed lights and shadows. The picture 

 thus produced would not be permanent, as subsequent expos- 

 ure would darken the light portion of the picture : to fix it, 

 the paper must be immersed in a solution which has the pro- 

 perty 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, re- 

 quiring 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 discovered 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 soas to be capable 

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



