A Manual Of Photography

CHAPTER V. THE PHOTOGRAPHIC PROCESSES ON PAPER OF SIR JOHN HERSCHEL. The researches of Sir John Herschel have been principally directed to the investigation of the physical laws which regulate the chemical changes we have been consideiing. is ana^s of the prismatic spectrum have been most complete, and as tar as they have been carried 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 bee engaged in investigations of tins high order, e multitude of his experiments, been successful in producing several processes of great beauty. I here are no any v ‘ be regarded as peculiarly sensitive-they are indeed for -the most part rather slow—but the manipulation required is of the easiest character, and the results are most curious and instructive. The philosophy which is for ever united with the scientific investigations 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 commu- nication to the Royal Society, and linked together by my own remarks in sucli a manner as it is hoped will be mo . understood by the unscientific amateur. Section I.—Cyanotype. The processes in which cyanogen is employed are so called. Sir John Herschel thus introduces the subject of his expeii- ments with these salts:-“ I shall conclude this part of my sub- ject by remarking on the great number and variety of substances which, now that attention is drawn to the subject, appear to be photographically impressible. It is no longer an insulated and anomalous affection of certain salts of silver or gold but one which, doubtless, in a greater or less degree pervades all nature, and connects itself intimately with the mechanism by which chemical combination and decomposition is operated. general instability of organic combinations might lead us to