206 THE CHROMATYPE. SECT. XXIV. 



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 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 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 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 ammonia-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 white and purple photograph results. 

 It is vain to attempt to describe the various beautiful effects 

 which Sir John Herschel obtained from chemical compounds, and 

 from the juices of plants ; the juice of the red poppy gives a 

 positive bluish purple image, that of the ten-week stock a fine 

 rose colour on a pale straw-coloured ground. 



Pictures may be made by exposure to sunshine, on all com- 

 pound substances having a weak chemical affinity ; but the 

 image is often invisible, as in the Daguerreotype, till brought out 

 by washing in some chemical preparation. Water is frequently 

 sufficient ; indeed Sir John Herschel brought out dormant photo- 

 graphs by breathing on them, and some substances are insen- 

 sible to the action of light till moistened, as for example, gum 

 guaiacum. Argentine papers, however, are little subject to the 

 influence of moisture. The power of the solar rays is augmented 

 in certain cases by placing a plate of glass in close contact over 

 the sensitive surface. 



All these various experiments, though highly interesting, have 

 now been superseded. It was found that paper did not always 

 answer for photography, on account of imperfections in its 

 structure ; silver plates were too expensive ; and glass was 



