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Refractive Index or Index of

medium, glass, refraction and plate

REFRACTIVE INDEX or INDEX OF REFRACTION.—When a ray of light passes obliquely from a rare to a dense medium it is refracted to a certain extent, varying with the medium employed, as the sine of the angle of incidence always bears an invariable ratio to that of the angle of refraction, for the same ray and the same medium. This ratio is termed the refractive index of that medium.

The following table gives the refractive indices of a few well known substances : Gas. Index of refraction.

Air I'000294 Oxygen r000272 Hydrogen 1 •00o138 Liquid. Index of refraction for mean yellow ray.

Linseed oil I'485 Oil of turpentine I'457 Alcohol 1.372 Ether r358 Water Glycerine 1'470 Solid Index of refraction for mean yellow ray.

Diamond 2.47 2.

Sulphur Flint glass. I.575 Rock crystal. I.547 Canada balsam 1.532 Crown glass .{ 1.525 1.534 Plate glass t 1 514 • Iceland spar. 1.654 Ice 1.315 REFRANGIBILITY.—The property which rays of light possess of being bent out of a straight line when they pass from one medium to another of different density.

RE-GILDING.—Photographic picture frames may be re-gilt after the following method given in " Spon's Workshop Recipes ": —Wash the frame well with a sponge and clean water, and when dry give it two coats of water-gold size mixed with some warm thin size from dry hide or parchment. When dry rub over with fine sand-paper, and it is then ready for gilding with

tha gold leaf. After acouple of days it can be finished with an agate burnisher.

REGISTER.—The register of a camera is the correspondence in position between the focus ing screen and the surface of the sensitive plate or film afterwards introduced. This, especially where lenses of large aperture are used, should be very carefully tested. The most effective method of doing this is by focusing a distant object, such as a signboard with sharply defined letters on it, using the full aperture of a rapid lens. A focusing glass should be employed if the operator does not possess good eyesight. The focusing screen is then removed, and each side of the different double plate-holders is brought into position, having a piece of ground glass fitted where the dry plate is usually placed. The two slides are then withdrawn, and the image exam ined. If perfectly sharp the register is correct. Another method is to expose a plate and examine for sharpness, but this is also a test for chromatic aberration in the lens.