PHOTOGICAPHY (Gr. phos, light, and graph°, I write). From the following brief sketch of the history of this art, it will be apparent that its present advanced fain has resulted from the combination of various discoveries in reference to the nature and prop erties of light made by investigators at different periods. Each inquirer has availed him self of the results obtained by previous students, adding to the common stock the results of his own investigations. The progress has been far more rapid than in most of the sciences which have been built up in a similar manner. Like other branches of chemis try, it owes its origin to the alchemists, who, in their fruitless researches after the phi losopher's stone and elixir vita?, produced a substance to which they gave the name of tuna cornea, or horn silver, which was observed to blacken on exposure to light. This property of the substance constitutes the leading fact upon which the science of photog raphy is based. More recently, the illustrious philosopher Scheele made experiments with the substance in question, with a view to determine the effects produced upon it by different rays in the solar spectrum. His words are these (published in 1777): " Fix a glass prism at the window, and let the refracted sunbeams fall on the floor; In the colored light put a paper strewed with Jana cornea, and you will observe that the horn silver grows sooner black in the violet ray than in any of the other rays." Still more recently, the names of Wedgwood and Davy (1802), and of Nicpcc and Daguerre from 1814 to 1839, occur as followers in the path indicated by Scheele and the earlier savans; and in the early months of the year 1864, the attention of the photographic. society of London was occupied by the endeavorto establish the authenticity and true photographic character of some pictures found in the library of Matthew Boulton, and believed to he true sun-pictures by James Watt, the celebrated engineer; thus offering great probability that the mind which produced the wonders of steam-power, Lad also been engaged in the same investigations which have resulted in the present wore extensive development of photographic science. Most of the experinn uts alluded to may be said to have been
based upon the fade, that the salt of silver, called by the ancients boot cornea, and by modern chemistschloride of silver, is highly sensitive to the influence of light. But such observers must have beeniully aware that this substance is not the only one affected by light, for it had been long noticed that the light of the sun does not fall agog our surface without leaving traces of its action thereon. -It cannot be absorbed or reflected without in some way modifying the structure and properties of the exposed surface. Even the brick and stone of which our houses are built become blanched by its influence, and those portions on winch the shadows of trees or other detached objects fall are perceptibly darker than those exposed to its full force; Nt ith the knowledge, therefore, of this all pervading influence before their minds, the investigations of scientific photographers have been directed to the production of surfaces either of metal, paper, or glass, so imbued with chemical substances as to possess a maximum amount of sensibility to this subtle agent —light.
There seems but little doubt that some of the acute-minded men who investigated the phenomena of the influence of light must have made use of *the beautiful invention of Baptista Forta of Padua. known as the camera obscura (q.v.); for the pictures of natural objects formed on the inner surface of this instrument would readily suggest its use in combination with the luna cornea. In the researches of a later period, the camera was used. The earlier attempts by its means failed, however. owing to the want of a power of fixing the images produced by the lens—i.e., of dissolving the unchanged tuna cornea, without acting on the reduced silver of which the image was formed. That want hov Mg, by mean's of chemical investigation, been supplied, the science of photography has become firmly established in its principles, and the practice of it as an art is diffused all over the civilized world.