This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Ammonium Sulphydrate, or Sulphuret of Ammonia. Colorless, crystalline masses; turning rapidly yellow on exposure to air. Soluble in water. Used as a blackening agent with both the lead nitrate intensifier and the mercuric chloride intensifier. It is usually prepared in solutions by passing hydrogen sulphide into ammonia water. It is strongly alkaline toward litmus paper.
Amyl-Acetate - C7H14O2 .
Colorless, transparent liquid; aromatic, ethereal odor (smells like pears). Use: A solvent for celluloid. Mixes with alcohol or ether, but not with water. Celluloid dissolved in amyl-acetate makes an excellent varnish; also valuable for mending celluloid dishes or articles. (See Varnish, Celluloid.)
(See Astigmatism; also Lens, Anastigmat.)
In photography a narrow angle is one embracing a view of less than 40 degrees, and is especially valuable for making artistic pictures.
In photography a normal angle is one embracing a view of from 40 to 65 degrees.
The angle formed by the line of refraction and a line drawn from the point of contact perpendicular to the plane or surface on which the refracted ray impinges.
The amount of subject shown by a lens on the ground-glass. A lens embracing an angle of about 35 degrees gives the most truthful picture, as the eye cannot take in a greater angle of view - such a lens has long focus. Exaggerated perspective always results when wide-angle lenses are used - objects in the foreground look large and out of proportion, while those more distant appear dwarfed. This distortion is only apparent, for if a picture be taken with a 4-inch lens and viewed at a distance of 4 inches from the eye, correct perspective will result.
A term applied to certain forms of lenses so constructed as to embrace a greater amount of view than the ordinary lens by having a very short focus in comparison to the base line of the plate. Wide-angle lenses are of value when photographing in narrow quarters, interiors, etc.
A chemical term signifying that a substance is absolutely free from water.
A colorless liquid. Used in preparation of aniline colors, also in orthochromatizing plates.