SSCT. xxiv.] THE CALOTYPE. 225 



gum guaiacum acquires a green colour in the violet and 

 blue rays, ,nd resumes its original tint in the red. No at- 

 tempt had been made to trace natural objects by means of 

 light reflected from them, till Mr. Wedgewood, together 

 with Sir Humphry Davy, took up the subject : they pro- 

 duced profiles and tracings of objects on surfaces prepared 

 with nitrate and chloride of silver, but they did not succeed 

 in rendering their pictures permanent. This difficulty was 

 overcome in 1814 by M. Niepce, who produced a permanent 

 picture of surrounding objects, by placing in the focus of a 

 camera obscura a metallic plate covered with a film of 

 asphalt dissolved in oil of lavender. 



Mr. Fox Talbot, without any knowledge of M. Niepce's 

 experiments, had been engaged in the same pursuit, and 

 must be regarded as an independent inventor of photography, 

 one of the most beautiful arts of modern times : he was the 

 first who succeeded in using paper chemically prepared for 

 receiving impressions from natural objects ; and he also 

 discovered a method of fixing permanently the impressions 

 that is, of rendering the paper insensible to any further 

 action of light. In the calotype, one of Mr. Talbot's most 

 recent applications of the art, this photographic surface is 

 prepared by washing smooth writing-paper, first with a so- 

 lution of nitrate of silver, then with bromide of potassium, 

 and again with nitrate of silver, drying it at a fire after each 

 washing ; the paper is thus rendered so sensitive to light, 

 that even the passage of a thin cloud is perceptible on it, 

 consequently it must be prepared by candle-light. Por- 

 traits, buildings, insects, leaves of plants, in short every ob-* 

 ject is accurately delineated in a few seconds, and in the, 

 focus of a camera obscura the most minute objects are so 

 exactly depicted that the microscope reveals new beauties. 



Since the effect the chemical agency of light is to destroy 

 the affinity between the salt and the silver, Mr. Talbot found 

 that, in order to render these impressions permanent on paper^ 

 it was only necessary to wash it with salt and water, or with 



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