The Roman numerals refer to Volume. The number following refers to paragraphs in all cases, excepting where the word "Page" precedes the number. When a subject is exhaustively treated upon in a volume, simply the volume number is given; reference should then be made to the index of that volume, where the page or paragraph numbers for the various sub-divisions of the subject will be found. Our authority for chemicals, symbols, description and solubility of chemicals given herein is based entirely upon MERCK'S CHEMICAL INDEX.

Aberration

A defect in a lens causing it to render a distorted image, and one lacking sharpness. {See Lens Tests.)

Chromatic Aberration

Caused by the different color rays of light coming to a focus in different planes. A defect found in all lenses, formed of a single piece of glass. (See Lens Tests.)

Spherical Aberration

Caused by the rays of light refracted by the margin of the lens meeting at a different point in the axis than those rays that are refracted from the center. This defect occurs principally in the single lenses. May be modified by using a suitable stop, or diaphragm, which cuts off the marginal rays of light. (See Lens Tests.)

Abrasion Marks

Black lines and markings on the surface of prints due to pressure on the gelatin film and, especially, scratching against the edges and corners of other pieces of paper, when taking from package. Easily removed by rubbing the dry print with alcohol.

Accelerator

A chemical used in a developer to quicken its action in bringing out the latent image. Example: Sodium, or potassium, carbonate.

Accessories

A term applied to apparatus, and especially to chairs, scenery, etc., in the studio.

Acetone

(CH3)2CO - II, 577, 583, 588, 597.

Clear, colorless liquid; fragrant, mint-like odor; sharp, biting taste. Mixes easily with water, alcohol and ether. Is very inflammable, therefore must be kept away from the flame. Readily dissolves fats, resins, camphor, celluloid, etc. In combination with other solvents it is used in the preparation of varnish for negatives. Within recent years it has been brought into use as an accelerator, in which connection the gelatin film is neither colored nor altered, neither does it tend to soften the sensitive film. This latter property is of particular interest to those working in warm climates. When acetone is employed the dark-room must be well ventilated, or it will produce violent headache.

Acetone Sulphite

White, floury, granular powder. Readily soluble in water. Keeps well. Used in developers and fixing baths as a preservative, taking the place of sodium sulphite, etc. It exercises a restraining action in the developer. Can be used as a blackening agent for intensification with chloride of mercury. It is unlike acetone, as it is not sufficient in itself to make a developer active.

Acetylene

A colorless gas burning with a bright flame. Produced by action of water on calcium carbide. It makes an excellent illuminant for enlarging, etc. It is poisonous, and explosive when mixed with air.