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light, plate, oil, exposed, metal, niepce and lavender

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NICOL'S PRISM—An instrument for polarizing light, named after the inventor. It is formed from a rhombohedron of Iceland spar thrice as long as its diameter. This is bisected in the plane, which passes through the obtuse angle. The new faces, being polished, are cemented together again by Canada balsam. A ray of common light, entering the Nicol prism at one end, is divided into two opposite polarized rays, the ordinary and extraordinary. When these rays meet the Canada balsam cement, the ordinary ray undergoes total reflection from this surface, and is sent out of the field at the side, while the extraordinary ray passes through alone. The emergent ray is therefore polarized in one direction only.

NlePCE'S PROCESS, or NIEPCEOTYPE.—One of the earliest photographic processes discovered by Niepce. He found that bitumen became insoluble by the action of the light. A metal plate, coated over with a thin film of bitumen (dissolved oil of lavender), was exposed to the image in the camera obscura, and the bitumen became insoluble in proportion to the intensity of the light by which the various parts of the image were produced. This effect is, we know, due to the oxidation and hardening of the resinous substance. After removal from the camera the exposed plate is steeped in a mixture of oil of lavender and petroleum ; the soluble portions remaining are dissolved away. The shadows of the image are thus represented by bare portions of the metal plate, the insoluble resin remaining representing the lights and high lights. It will be clear that to get the best effect the polished metal representing the shadows should be darkened. For this purpose Niepce employed iodine and various other chemicals. Niepce, in a statement made in the year 1829, thus describes his process :—" The discovery which I have made, and to which I give the name of ' heliography,' consisting in producing spontaneously by the action of light, with gradations of tints from black to white, the images received by the camera obscura. Light acts chemically upon bodies. It is absorbed, it combines with them and communicates to them new properties. Thus it augments the natural consistency of some of these bodies ; it

solidifies them even, and renders them more or less insoluble according to the duration of inten sity of its action. The substance which has succeeded best with me is asphaltum dissolved in oil of lavender. A tablet of plated silver is to be highly polished, on which a thin coating of the varnish is to be applied with a light roll of soft skin. The plate, when dry, may be immediately submitted to the action of light in the focus of the camera. But, even after having been thus exposed a length of time sufficient for receiving the impressions of external objects, nothing is apparent to show that these impressions exist. The forms of the future picture remain still invisible. The next operation, then, is to disengage the shrouded imagery, and this is accom plished by a solvent, consisting of one part by volume of essential oil of lavender and ten of oil of white petroleum. Into this liquid the exposed tablet is plunged, and the operator, observing it by reflected light, begins to perceive the images of the objects to which it had been exposed grad ually unfolding their forms. The plate is then lifted out, allowed to drain, and well washed with water." Niepce further adds, " It were, however, to be desired that by blackening the metal plate we obtain all the gradations of tone from black to white. The substance which I now em ploy for this purpose is iodine, which possesses the property of evaporating at the ordinary tem perature.

NIGHT PHOTOGRAPHY.—Very successful photographs have been made at night time using the illumination from the electric lights. Figs. 296 and 297 are examples made by Mr. W. A. Fraser. This gentleman thus describes his method : " The first requisite is a strong weather-proof box cam era. The one I use for this work is an old style Scovill detective, long ago laid on the shelf, but it struck me when thinking the matter over, that its strength and solidity, made it a very suitable instrument for this purpose, and much work done with it during the past winter has confirmed me in this opinion.

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