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Photo Crayon

plate, glass, tin, surface, wire, pure, sensitive and dark

PHOTO CRAYON PROCESS.—A process introduced by Sarony about the year 187o. It contained in making a photographic transparency on glass, which was afterwards backed up with paper, on which a number of lines, hatchings, or stippling were lithographed, giving the portrait the appearance of a crayon work.

A similar process adopted by Henderson is the following :—A piece of course ground glass is laid over a portrait, and the glass worked on with a lead pencil or crayon all round, and, if necessary, over the lower part of the figure. The ground glass when viewed by transmitted light will then represent beautiful broken up granular markings. A thin negative is then made from this ground glass from transmitted light, reduced or enlarged as required. This negative to be superimposed with the negative to be enlarged or reproduced, which when developed, will contain all the worked up effects on the ground glass.

PHOTO ELECTRICITY.—The generation of electromotive forces and currents by the action of light on various metallic plates having their surfaces prepared by special processes. The most recent experiments in this direction have been made by Professor G. M. Minchin.* He uses two kinds of cells. One of them consists of a glass tube three or four inches long, in which is some pure methyl alcohol from oil of wintergreen, covering the two plates ; platinum wires, sealed into the tube by heat, pass through the glass to the plates. This smaller plate is of absolutely clean, pure tin, a quarter of an inch long, a sixteenth of an inch broad. The larger and sensitive plate is one inch long and one-eighth of an inch broad. This one also is of per fectly pure tin, for any impurity, especially any trace of copper, promotes failure in the results. The plate is first cleaned with sodic hydrate, and afterwards with dilute hydrochloric or hydro fluoric acid ; then it is laid upon a horizontal arm of porcelain so bent at the other end that, on raising from below a dish containing liquor, the said liquid covers the plate. By this method the — - - - — — second plate, or the one which has to be covered with a sensitive film, is immersed in the follow ing solution :— Distilled water soo c.c.

Nitric acid 3 44 • Nitrate of ammonia is The plate is left in the liquid about four minutes, and becomes covered uniformly with a whitish deposit. The solution is then removed by lowering the dish, and the under surface of the horizontal porcelain support is dried with blotting paper. This dried under surface is then uni formly heated with a spirit flame moved about underneath until the liquid above has evaporated; the surface of the tin plate will then present a dirty slate color. As the heating is continued,

a point is reached at which a dark shadow passes over the whole surface of the plate ; if the heating be now stopped, sensitive plate is produced, but not one of the maximum sensitiveness. Upon continuing the heat the surface will change into a perfectly white one, and the heating should be continued until the thin vapor or smoke which is given off ceases to appear, and until the smell of nitrous acid entirely disappears. Care must be taken not to melt the tin in this process, and when the treatment is complete, the plate should be plunged into methyl alcohol from pure oil of wintergreen A fine platinum wire has first to be fixed to the top of the plate, either by means of solder with a low melting point, or by passing a wire through a little hole at the top of the plate and then bending the end of the wire back over the top edge ; the latter plan is found to give sufficiently good contact for practical purposes. The clean, plain tin plate is sealed to the bottom of the tube by means of its platinum wire ; after the methyl alcohol and the other plate are inserted, the upper part of the tube, with the wire from the second plate pass ing through it, is sealed by heat. All this may be done in daylight. The complete cell has to be left from two to five hours in the dark before it will exhibit its maximum powers.

The tin plate thus rendered sensitive, and mounted in a cell as described, when exposed to good diffused daylight will exert an electromotive force—E M.F. in electricians' language—of half a volt or more, as exhibited by means of the quadrant electrometer, and it will yield a steady stream of electricity for three or four hours, after which the E.M.F. falls off.

Supposing an exposure not to have been too long, the cell will gradually recover itself in the dark ; if it be not exposed for more than ten minutes or so at a time it will recover itself in the dark ; one of the cells which has been used only in the latter manner has been so employed by Professor Minchin for four years.

Photo electricity is yet in its infancy, but there are many who believe that it has a great future before it. Among the many possibilities are mentioned* a scientic comparison of the relative values of two different lights, the solution of the problem of telephotography, or seeing at a distance, also the question of utilizing the energy of the solar rays for performing useful Work by means of photo-electric batteries.