196 THE CHROMATYPE. Scr. XXiV. 



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 vapor, 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 de- 

 gree of illumination, the shading of the picture is per- 

 fect. 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 color is destroyed, and in order to render 

 the mercury permanent, the plate must be exposed a 

 few minutes to nitric vapor, then placed in nitric acid 

 containing copper or silver in solution at a temperature 

 of 61| 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. 

 Color alone is wanting ; but the researches of Sir John 

 Herschel give reason to believe that even this will ulti- 

 mately be attained. 



The most perfect impressions of seaweeds, leaves of 

 plants, feathers, &c., may be formed by bringing the 

 object into close contact with a- sheet of photographic 

 paper, between a board and plate of glass ; then ex- 

 posing the whole to the sun for a short time, and after- 

 ward fixing it by the process described. The colors of 

 the pictures vary with the preparation of the paper, by 

 which almost any tint may be produced. 



In the chromatype, a peculiar photograph discovered 

 by Mr. Hunt, chromate of copper is used, on which a 

 dark brown negative image is first formed, but by the 

 continued action of light it is changed to a positive 

 yellow picture on a white ground ; the farther effect 



