Photography

Positive materials may be used for transparencies, in which case they are lantern slides on glass or positive films (especially for motion picture photography [q.v.] ). These materials are prepared in exactly the same way as negative materials, the emulsion being modified to give the results desired. Usually, however, negatives are printed upon sensitized paper. Printing papers at the present time are generally coated with a sensitive emulsion prepared either with silver bromide, bromide papers, or with silver chloride, gas light or developing-out papers.

Previously, the most usual types of sensitive paper used were prepared with an emulsion containing free soluble silver com pounds and were printed under the negative in strong light until the image was complete—printing-out papers. No development was required with these papers, which are, however, almost ob solete now except for the preparation of proofs submitted by professional photographers to customers.

Many other varieties of printing processes are known and some are still being used to a limited extent.

Photography was discovered by no one man. It was the out come of the early observations of the alchemists and chemists on the action of light, a subject that belongs strictly to the domain of photochemistry (q.v.). Although the blackening of silver salts wa3 known in 1565, it was not until 1727, when J. H. Schulze, of Germany, used a mixture of silver nitrate and chalk under sten ciled letters, that it was definitely recognized that this ,darkening action was due to light and not to heat. These experiments were important in that, in conjunction with those of K.W.Scheele,i777, they led to the experiments of T. Wedgwood with silver nitrate on paper and leather in 1802, which were reported to the Royal Institution of London with the suggestion that silver chloride was more sensitive. These were practically failures because of the very long exposures required, and no means were then known of removing the unaffected material and thus stabilizing the image against light.

Camera.

Leaving for the time being the question of the sen sitive salts, we may turn to the evolution of the camera. This was the outcome of the old camera obscura, the invention of which is usually ascribed to Baptista Porta, 1553, though its principle had been indefinitely described by Alhazen, i ioo; Roger Bacon, 1267; and others. Leonardo da Vinci, who died in 1519, described and pictured a camera obscura in an unpublished manuscript. In 1550 J. Cardan suggested the use of a speculum or concave

mirror in front of the instrument, and D. Barbaro, 1568, pro posed convex lenses and the use of a diaphragm to secure greater sharpness of the images. E. Danti, 1573, corrected the reversed image by means of a mirror behind the lens, a device still in use. F. Risner, who died in 1580, described in his works published in 1606 the methods of enlarging and reducing, and a portable box in lieu of the cumbrous fixed hut in use until then. Porta in his second edition, 1589, of which an English translation was pub lished in 1658, was the first to introduce the use of the Convex lens in the camera. Kepler, the famous astronomer, in his work on optical astronomy, 1604-1611, described the use of a concave lens behind the convex to obtain larger images, and thus antici pated the telephoto lenses of the present day. In J. Zahn's book, •1665, there was described a portable camera obscura with two or three lenses, to secure greater brilliancy of the image, and side wings to shield it from extraneous light ; thus with the reversing mirror he antedated a type of modern camera. At the beginning of the eighteenth century the portable camera obscura had be come a regular article of commerce and was used to obtain sketches from nature.

The Beginnings of Photography.

A. Senefelder discovered lithography in 1796, but, though it was introduced in Paris in 1802, it was not until 1813 that it became a success and a fashion able hobby. From this date until 1817 J. Nicephore Niepce was engaged in an examination of the French natural stones to find one suitable for the process. Since he was unable to draw, his son, Isidore, undertook this work, and when the latter was called for military service, Niepce was impelled to produce the images auto matically. In conjunction with his brother Claude, Niepce first essayed photography with silver chloride on paper. The light sensitiveness of iron, manganese, acids, and other compounds was tested, stone, metal, and paper being used as supports. Finally, guaiacum resin was tried and then asphaltum, or bitumen of Judea, which had been in common use as an etching ground since the days of Rembrandt. This, becoming insoluble in its usual solvents by the action of light, gave not only a resist for the etching of metal plates but also transparent images on glass. A successful result was obtained in 1822, and thus it may be said that the first permanent photograph was made in that year by Niepce.