The Ambrotype succeeded the Daguerreotype, and was produced by making a very thin negative by under exposure on glass, using the collodion process, and, after drying, backing the glass with black asphaltum varnish or black velvet, causing the dense portions of the negative to appear white by reflected light, and the transparent portions black. Such pictures were quickly made, and were much in vogue forty years ago, but are now obsolete. A modification of the ambrotype, however, still survives in what is known as the “tin-type” or “ferro-type.” In the tin-type the collodion picture is made directly upon a very thin iron plate, covered with black enamel, which both protects the plate from the action of the chemicals in the bath, and forms the equivalent of the black background of the ambrotype.
Silver Printing.—A sheet of paper, previously treated with a solution of chloride of sodium and dried, is sensitized in an alkaline bath of nitrate of silver. When the paper is exposed under a negative, the light through the transparent parts of the negative reduces the silver, converting the chloride, it is supposed, into a metallic sub-chloride of silver which becomes dark or black, and constitutes the main portion of the picture. The image is then fixed by dissolving out the chloride of silver unaltered by light in a bath of hyposulphite of soda. After fixation, the image is well washed in several changes of water to eliminate all traces of the hyposulphite of soda and prevent the subsequent fading of the darkened portions of the picture and the yellowing of the whites. If the printed image is immediately fixed, it will have a red color. To avoid this it is washed first in water and then immersed in a chloride of gold toning bath and fixed.
The Platinotype Process is one in which potassium chloroplatinite and ferric oxalate are converted by light into the ferrous state, and metallic platinum is reduced when in contact with the ferrous oxalate of potash solution. The unacted upon portions are dissolved out by dilute hydrochloric acid, leaving a black permanent image. This process is characterized by simplicity, sensitiveness in action, permanence of print, and a peculiarly soft and artistic quality in the picture. British Patent No. 2,011, of 1873, to Willis, is the first disclosure of the platinotype.
Carbon Printing is a process in which lampblack or other indestructible pigment is mixed with the chemicals to render the photograph more stable against fading from the gradual decomposition of its elements. Mungo Ponton, in 1838, discovered the sensitive quality of potassium bichromate, which led up to carbon printing. Becquerel and Poitevin, in Paris, in 1855, were the first to experiment in this direction, and Fargier, Swan, and Johnson were successors who made valuable contributions.
Emulsions.—A photographic emulsion is a viscous liquid, such as collodion or a solution of gelatine, containing a sensitive silver salt with which the glass plate is at once coated, instead of coating the plate with collodion or gelatine, and then immersing it in a sensitizing bath. The desirability of emulsions was recognized as early as 1850 by Gustave Le Gray, and in 1853 by Gaudin. Collodion emulsion with bromide of silver was invented by Sayce and made known in 1864. In 1871 Maddox published his first notice of gelatine emulsion, and in 1873 the gelatine emulsions of Burgess were advertised for sale. In 1878 Mr. Charles Bennett brought out gelatino-bromide emulsion of extreme sensitiveness, by the application of heat, and from this time gelatine began to supersede all other organic media.
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.