SECT, xxiv.] THE CHROMATYPE. 227 



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 per- 

 manent, 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 Gl^ 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 alone 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, 

 feathers, <kc., may be formed by bringing the object into close 

 contact with a sheet of photographic paper, between a board 

 and plate of glass ; then exposing the whole to the sun for a 

 short time, and afterwards fixing it by the process described. 

 The colours of the pictures vary with the preparation of 

 the paper, by which almost any tint may be produced. 



In the chroniatype, 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 of light is checked by washing 

 the picture in pure water. 



In cyanotypes, a class of photographs discovered by Sir 

 John Herschel, in which cyanogen in its combinations with 

 iron forms the ground, the pictures are Prussian blue and 

 white. In the chrysotype of the same eminent philosopher, 

 the image is first received on paper prepared with the am- 

 monia-citrate of iron, and afterwards washed with a neutral 

 solution of gold. It is fixed by water acidulated with sulphuric 

 acid, and lastly by hydriodate of potash, from which a 



