PHOTOGRAPHY. The art of producing the appearance of objects and fixing them by means of light on a previously sensitized surface. and the reproduction of the image thus obtained, by various processes, on glass, paper, and other terials. The first photographic camera was in reality a darkened room to which light was ad mitted through a single small hole in the window shutter. This was the camera obscura of iliam battista della Porta, au Italian philosopher, in vented by him in the last half of the sixteenth century. When the sun shone brightly. a faint inverted image of a landscape could be seen on the whitened surface of the wall within. The camera obscura of the present time differs very little from Porta's invention ex cept that it contains a lens in the apex of the roof, with a reflecting mirror above it. in order to throw the light from all the surround ing landscape down through the lens and thus form a panorama on the surface of a table placed in the middle of the room. J. H. Schultze. a Cler man, who has been called 'The Columbus of Pho tography,' obtained the first actual photographic copies of writing as early as 1727. by placing the written characters upon a level surface previous ly prepared with a mixture of chalk and silver ni trate. The rays of light passing through the translucent paper blackened the silver compound underneath, except where it was protected by the opaque ink forming the letters themselves, and thus a white copy upon a black ground was ob tained.
Scheele in 1777 demonstrated that silver chlo ride became black quickest by the action of the violet rays of the solar spectrum, this showing that the rays of light were not equally active. A few years later Professor Charles projected the shadow of a head by means of strong sunlight on a sheet of white paper which had been made sensitive by chloride of silver and obtained a white profile on a darkened surface; but he was unable to fix the image upon the sensitive paper. :Meanwhile the members of the Lunar Society, in cluding Samuel Parr, James Watt, Josiah Wedg wood. and others. met regularly at the house of Matthew Boulton at Soho and discussed their ex periments. some of which were on the action of light, and it has been shown that an artist named Eginton, in the employ of BouIt on. made and sold numerous collies of well-known paintings at very low prices. Thomas Wedgwood, to whom is due the honor of being the first to produce pic tures by the action of light on a sensitive surface. published his paper giving "an account of the method of copying paintings upon glass, and of making profiles by the agency of light upon ni trate of silver, with observations by H. Davy," in the Journal of the [loyal Inslilute for dune, 1802. To Davy credit is due for his discovery that silver chloride was more sensitive than the nitrate; but, notwithstanding his continued in vestigations, Davy was unable to find a means by which the failing out of the pictures could be prevented. Other investigators took up the sub ject later, among whom may be mentioned Joseph Nicpce and Daguerre in France, and Wil liam H. F. Talbot in England. The flrst named, in 1814, succeeded in producing perma nent pictures by a process which he called heliog raphy. It consisted in coating with bitumen a piece of plated silver or glass. which was then exposed in a camera obscura during 4 to h hours. Those portions of the bitumen which were acted on by the light became insoluble, and by remov ing the parts unacted upon by certain essential oils in which bitumen was soluble, the shadows of the image were removed, and the lights were represented by the insoluble bitumen that re mained on the plate. This process was therefore the forerunner of our modern photo-mechanical methods. In 1824 Daguerre began his experi ments, which led to the invention of his cele brated process. He formed a partnership with Niepee in 1829. and ten years later they an nounced the invention of the daguerreotype pro cess, the principle of which had been discovered by Daguerre in 1832. The consisted in ex posing a metal plate covered with silver iodide for twenty minutes in a photographic camera. after which the plate was transferred to a dark room and exposed to the vapor of mercury.
developed the latent image, the latter being then made permanent by means of a solution of sodium chloride. Sir John Herschel. who devoted consid erable attention to this subject. soon after an nounced the greater suitability of sodium hyposul 'Mite for dissolving the haloid salts of silver and consequently the hyposulphite was adopted as a fixing agent. Daguerrc received a pension of Wit from the French Government on gus' lu, I:7,39, in consideration of which the de tails of his process were given to the world, States of this invention reached the States in 1s39 through Samuel F. 11. Morse, who communicated it to his colleague in New lork l'niversity, John \V. Draper (q.v.), by whom the first smffight picture of a human face (that of his sistel Dorothy Draper) was made in IS-10. Meanwhile Talbot, in England, had been pur suing investigations from a different view-point, and on January 31, 1s30, be presented heftily the Royal Society a paper on "Pathogenic Drawings." These he produced by dipping writ ing paper in a solution of sodium chloride. dry ing. and then transforming the sodium chloride into silver chloride by passing the paller through a solution of silver nitrate. With this paper, which was extremely sensitive to light, he was able to produce a negative that became black by the action of sunlight except where it was cov ered by some opaque object. which he desired to reproduce, and in turn from this negative. by another i.xposnre. a number of positive prints could be obtained. which were fixed by potassium ',nimble. Two years later Talbot patented his 'calotype' pnicess, in which the negative was Iola:lined by coating the surface of the paper with silver iodide, then washing it over with a mix inn' of silver nitrate with gall k' and acetic acids, after which it was exposed in a camera to the ob ject lie wished to copy. The invisible picture thus obtained was developed by silver aceto-ni trate and gain(' acid. and fixed with potassium bromide. Minor inventions and improvements followed the introduction of Talbot's process, among which the most important was the of albumen, recommended by Nierce de St. Vic tor as a film containing haloid salts which he flowed upon the surface of the glass, The development of the modern rapid,processes of photography may be said to have begun with the introduction of the dry collodion process by Scott Archer in 1S51. This process consists in coat ing glass !dates with a film of containing soluble iodides nr bromides, which form a sensi tive silver compound when dipped in a solution of silver nitrate. The soluble collodion is pre pared by the action cotton of a mixture of nitric and sulphuric acids with a little water, or of a mixture of potassium nitrate with sulphuric acid. the resulting substance being dissolved in a mixture of alcohol- and ether. On evapora tion, the viscous solution leaves a film of collo dion. The exact proportions of the ingredients used must be varied at different seasons of the year, and must he suited to the character of the objects to be photographed. one variety being suit able for landscape views, another for portraiture, et c. The collodion is flowed over the glass plate, which must be perfectly clean, and when partially dry sensitized by dipping into a bath containing silver nitrate in the proportion of from 35 to 50 grains of the salt to an ounce of water. A very small quantity of potassium iodide is added to the bath. The sensitized plate is then exposed in the camera to the object to be taken. The action of the light on the sensitive silver salts changes them in such a way as to jiroduee a latent on the film. which is then developed by pouring i over the plate a solution of ferrous sulphate or pyrogallic acid. to either of which are added some alcohol and acetic acid, after which the image on the plate is made permanent or fixed by immer sion in a solution of sodium hyposulphite, potas sium cyanide, or some other liquid capable of dis solving the portion of the silver salts that has not been acted upon by the light. For the pro tection of the collodion film it is common to coat the negative with a clear and hard varnish.