Photography

Collodion Process.

Collodion, a solution of nitrocellulose in alcohol-ether, was first prepared by Maynard of Boston for medi cal purposes. Schi5nbein, the Swiss chemist, discovered nitro cellulose in 1846. To a solution of pyroxylin in ether and alcohol, Archer added a soluble iodide, usually with the addition of a little bromide, and coated a clean glass plate with the iodized collodion. In the darkroom, the iodized collodion was sensitized by immersion in a bath of silver nitrate and formed silver iodide with excess of silver nitrate. The plate was exposed wet in the camera, developed by pouring on a solution of pyrogallol contain ing acetic acid, and fixed with a strong solution of thiosulphate of soda for which cyanide of potassium was later substituted. Archer's collodion process was not patented, and in three or four years it displaced both Calotype and Daguerreotype.

The necessity for preparing the plates immediately before ex posure and developing them immediately after considerably limited the practice of photography. Spiller and Crookes sug gested bathing the prepared plates in a solution of a hygroscopic salt, so that they could be kept some time both bef ore and after exposure. This was followed by the use of hygroscopic sub stances of all kinds.

Collodion Emulsion.

In 1864, B. J. Sayce and W. B. Bolton described the preparation of an emulsion of silver bromide in collodion. With this preparation, the nitrates—the by-products of the formation of the silver halide from the nitrate and alkaline halide—were left in the emulsion. In 1874, W. B. Bolton showed that it was possible to wash these nitrates out, and his methods are still followed.

, Collodion emulsion was a practical advance on wet collodion but involved no improvement in the matter of sensitivity. Funda mental contributions to this were made by the introduction of alkaline development by Major Russell in 1862. A considerable increase of sensitiveness was obtained with collodion emulsion, but the full fruit of this was reaped only with the introduction of gelatino-silver bromide dry plates.

Gelatine Emulsion.

In 1871, Dr. R. L. Maddox made an emulsion of silver bromide in essentially the same manner as that used for making collodion emulsions but he replaced collodion by gelatine. The matter was followed up by other experimenters, among whom may be mentioned J. Burgess and J. Kennett. Ken nett placed on the market a dry, washed emulsion which photog raphers could dissolve in warm water and use as a coating on glass and thus produce their own plates.

A great amount of experimental work was at once commenced on gelatine emulsions, the records of which filled the photographic journals between 1873 and 1885. The by-product salts were re

moved by washing. Abney recommended the use of iodide in small quantity with the bromide and found that this made it pos sible to obtain faster emulsions with less fog. Digestion, or "ripen ing" as it was called, came into use—long digestion at low tempera tures being suggested by Bennett—in 1878, and digestion with ammonia was used in 1876 by J. Johnson and in 1879 by Monck hoven, who employed precipitation in ammoniacal solution as the basis of a process of manufacturing dry plates.

In 1877, the commercial plates of the Liverpool Dry Plate Company, Wratten and Wainwright, and B. J. Edwards were intro duced, and by 1879 comparatively rapid dry plates were available on the market similar in type to the slower varieties of plates used to-day.

After this period, amateurs gradually ceased their researches, and mass manufacture became general. Considerable increases in sensitiveness were made between 1890 and 1900, but since the latter date there has probably been little advance in the maximum sensitiveness that can be obtained, although materials of high sensitiveness are made with much greater regularity and with greater unifoimity than was the case formerly.

It is interesting at this point to record the advance made in photography in terms of the relative sensitivity of the process.

Colour Sensitivity.

The silver halides are sensitive chiefly to the blue, violet, and ultra-violet rays; hence, all other colours are reproduced as dark greys or blacks. In 1873, H. W. Vogel of Berlin discovered that the addition of certain dyes to the emul sion or immersion of the coated plates in a solution of dye in creased the sensitiveness to the less refrangible colours.

Following this, Waterhouse found that eosin sensitized col lodion emulsion, and shortly afterwards Clayton and Tailfer found that eosin would sensitize gelatine emulsions. They ob tained a patent for its use in England and France, and their plates were placed on the market under the name of "isochro matic" plates. In 1884, eosin was replaced by erythrosine, which was found by Eder to be a better sensitizer, and since that date erythrosine has been used almost exclusively for the use of the so-called "isochromatic" or "orthochromatic" materials. In 1902, Miethe and Traube of Berlin found that ethyl red, an isocyanine dye, gave strong colour sensitiveness as far as the orange of the spectrum, and in 1905, Homolka, working at the Hoechst Dye Works, discovered Pinacyanol, a dye of structure somewhat similar to ethyl red but which sensitizes very powerfully throughout the red.