Photography

principle is the same (fo-tog'ra-fe.) in all. [From Gr. phōs, light, and gra pho, I write, or gen nao, I produce.) Proce es for the production of pictures by means of light. There is reason to believe, however, that the effect is due to other than the luminous rays of the spectrum. We have already given an account of the Daguerreotype proce (q. v.). The principles of the photographic proce , properly so called, may be understood by ex-Phrase, per i men ting with a piece of paper which has been dipped in a solution of nitrate of silver and cautiously dried; if the light is carefully excluded, it remains white; but if it is exposed to light, it gradually darkens, until it at last becomes black. If, while it is white, any opaque or translucent object is laid upon it, so as wholly or partially to intercept the rays, a representation of the object is obtained on the paper. Where the Phrenitis, rays have been totally intercepted, it remains white; where none of them have been intercepted, it is dark; and where they have been partially intercepted, it is darkened to an extent dependent on the amount of light to which it has been exposed. To fix this picture, it must be washed in the solution of a hyposulphite, to remove the unaltered salt of silver, and prevent further darkening. Instead of paper containing only nitrate of silver, one which, after having been impregnated with that substance, has been dipped in a solution of common salt-to change the nitrate of silver into chloride-is employed; having been found more sensitive to the action of light. Photographic papers have been prepared also with other substances, which improve their delicacy, color, & c. If some of the prepared paper is put into the camera obscura, a negative picture will be produced; and if this is printed from-that is, if it is placed between another portion of the prepared paper and the light-a positive picture will result. Collodion (q. v.), carefully spread on gla , and rendered sensitive by some of the salts usually employed for the purpose, is found, in many cases, more convenient than paper. And by varying the proce a little, either a positive or negative picture can be had at pleasure. Albumen may be used instead of collodion, also waxed paper, . There are other methods termed the cat a lis so type, the ferro type, .; but they vary chiefly in the materials adopted to produce sensitivene , color, . The advantage of a negative picture is that a number of copies maybe made from it without repeating the original proce , whereas good copies cannot be obtained from a positive picture.Chromo-P. is that branch of the photographic art which relates to the production of photographs in their natural colors. Many attempts have been known to produce photographs in natural colors; this has been partially accomplished by Niepce de St. Victor, E. Becquerel, and others, and tolerably truthful representations of colored objects and even the solar spectrum have been exhibited by these experimentalists; but all attempts to render them permanent have hitherto failed; exposure to light gradually obliterates them. [s. 707]