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8 THE ADVANCE OF PHOTOGRAPHY
should be excluded by covering the head with a black cloth, as indicated in the diagram.
These images appear still more beautiful if, instead of a hole, a glass lens is substituted. This lens, at a certain distance, which is equal to that of its focus, casts a distinct image of distant objects, which is much better defined and clearer than that which is produced by a hole.
In this improved form the instrument was emplo}Ted by Wedgwood and Davy. Their idea was to fix on sensitized paper the image produced upon the screen. They fastened a piece of paper saturated with a silver salt upon the place of the image, and left it there for several hours — unfortunately without result. The image was not bright enough to make a visible impression upon the sensitized paper, or the paper was not sufficiently sensitive.
Action of Light on Asphalt — Nicophore Niepce. — It now became necessary to find a more sensitive preparation to catch the indistinct image ; and this was achieved by a Frenchman — Nicophore Niepce. He had recourse to a very peculiar substance, the sensitiveness of which to light was before unknown to anyone — asphalt, or the bitumen of Judaea. This black mineral pitch, which is found near the Dead Sea, the Caspian, and many other places, is soluble in ethereal oils — such as oil of turpentine, oil of lavender — as well as in petroleum, ether, etc. If a solution of this substance is poured upon a metal plate, and allowed to cover the surface, a thin fluid coating adheres to it, which soon dries and leaves behind a thin brown film of asphalt. This film of asphalt does not become darker in the light, but it loses by light its property of solubility in ethereal oils.
If such a plate, therefore, is put in the place of the image on the camera obscura, the asphalt coating will remain soluble on all the dark places (shadows) of the image, whilst on the light spots it will become insoluble. The eye, it is true, does not perceive these changes ; the plate,