_Dry Plates_ were a great improvement over the old wet process, with its tray for baths, its bottles of chemicals, and other accessories. Especially was this the case with out of door work, which heretofore had involved the carrying along of much unwieldy and inconvenient paraphernalia. With the dry plate process only the camera and the plates were needed, and this step marks the beginning of the spread of the art among amateurs, and the great snap-shot era of photography, growing into a distinct movement about the year 1888, has since spread over the entire world. The first practical dry plate process (collodion-albumen) was published in 1855 by Dr. J. M. Taupenot, a French scientist. Russell, in 1862; Sayce, in 1864; Captain Abney, for photographing the transit of Venus in 1874; Rev. Canon Beechey, of England, in 1875; Prof. John W. Draper, of the University of New York, and the Eastman Walker Company, of Rochester, were the chief promoters of dry plate photography. The practical introduction began about 1862 with the application of the alkaline developer. The progress of the photographic art may be approximately noted as follows: _Process._ _Time Required._ _Introduced._ Heliography 6 hours’ exposure 1814 Daguerreotype 30 minutes’ exposure 1839 Calotype or Talbotype 3 minutes’ exposure 1841 Collodion process 10 seconds’ exposure 1851 Collodion emulsion (dry plate) 15 seconds’ exposure 1864 Gelatine emulsion (dry plate) 1 second exposure 1878 _Mechanical Development._--The photographic camera is but an adaptation of the optical principles of the old camera obscura, which has been credited to various persons, including Roger Bacon in 1297, Baptista Porta about 1569, and others. The essential elements of the camera obscura are a dark chamber, having in one end a perforation containing a lens, and opposite it on the back of the chamber a screen upon which an image of the object is projected by the lens for the purpose of enabling it to be directly traced by a pencil. The photographic camera, introduced by Daguerre in 1839, adds to the camera obscura some means for adjusting the distance between the lens and the screen on which the image falls. This was accomplished by making the dark chamber adjustable in length by forming it in two telescopic sections sliding over each other, and in later years by the well-known bellows arrangement. A luminous image of any object placed in front of the lens is thrown in an inverted position upon the screen, which is of ground glass, to permit the image to be seen in focusing. When the proper focus on this ground glass is obtained a sensitive plate is put in the plane of this screen to receive the image. [Illustration: FIG. 203.--KODAK.]