HISTORY OF PHOTOGRAPHY." 



By Robert Hunt. 



I. The Photographic Processes on Paper of Sir John Hersciiei.. 



The researches of Sir John Herschel have been principally directe^l 

 to the investigation of the physical laws which regulate the chemical 

 changes we have been considering. His analyses of the prismatic 

 spectrum have been most complete, and as far as they have been car- 

 ried out go to prove the operation of forces other than those with 

 which we are acquainted. 



At the same time, however, as this philosopher has been engaged 

 in investigations of this high order, he has from the multitude of his 

 experiments been successful in producing several j^rocesses of great 

 beauty. There are not any which are to be regarded as peculiarly 

 sensitive — they are, indeed, for the most part rather slow — l)ut the 

 manipulation required is of the easiest character, and the results are 

 most curious and instructive. 



The philosophy which is forever united with the scientific investi- 

 gations of Sir John Herschel is too valuable to be omitted from any 

 description of the processes which he recommends. The following 

 quotations are, therefore, taken from his communication to the Royal 

 Society, and linked together by my own remarks in such a manner as 

 it is hoped will be most easily understood by the unscientific amateur. 



CYANOTYPE. 



The processes in which cyanogen is employed are so called. Sir 

 John Herschel makes the following remarks on the subject of his ex- 

 periments with the cyanides : 



I shall conclude this part of my subject by remarking on the groat numb(>r 

 and variety of substances which, now that attention is drawn to the subject, 

 appear to bo photographically impressible. It is no longer an insulated and 

 anomalous affection of certain salts of silver or gold, but one which doubtless, in 



" Extracts from A Manual of Photography, by Robert Hunt, fourth edition, 

 London and Glasgow. 1854. 



163 



