The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Letter-Press Printing
Light
Letter-Press Printing, Photographic Substitute for. See Automatic Printing. For particulars of Letter-Press block making see Fish Glue Process.
Levelling Slab. An even piece of glass, slate, or other material sufficiently thick not to bend when placed upon the levelling stand, which is preferably of a triangular form, having a fine screw at each foot to allow of the height being altered as required.
Lichtdruck. Synonymous with Collotype, which see.
Light is that principle which emanates from all luminous bodies, and the luminosity of such bodies is believed to be due to intensely rapid molecular vibration, which vibration is propagated in a supremely subtle elastic medium, termed the luminiferous ether. Light always travels in straight lines, unless deviated from its course by the action of some body through which it passes. Transparent substances allow light to pass through them, but when the incidence is oblique the course of the rays is deviated ; translucent bodies whilst allowing some rays of light to pass through reflect others; whilst opaque bodies reflect some rays and absorb others, but the rays passing on each side of the opaque substance still continue their course, and leave behind the body a space which is not permeated by light rays, or is only partially so. The sectional image of this space received on a screen is termed shadow. Shadows, how-ever, are not rigidly defined, as the shadow cast by any source of light other than a point may be regarded as an infinity of shadows partly overlapping, and the partial shade at the edges of a shadow is called the penumbra. The velocity of light is about 186,830 miles per second, and its intensity varies in inverse proportion to the square of the distance from the source. If rays of light fall upon a body which is opaque, and the surface of which is unpolished, certain portions of such light will be absorbed or enter to a certain depth, the remaining por-tion being reflected or scattered in all directions; but certain bodies absorb only certain of the constituent rays of light, and reflect one or more of the others, and thus we arrive at the colours of objects. For example, an object which absorbs all the primary colours but red will appear red ; one that absorbs all
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