Printing-Out Paper, or P. O. P

A name given to gelatino-chloride paper, due to the image printing out fully when submitted to the action of light.

Backing for Prints

A paper coated with a collodion emulsion, lacking the sensitive salts, pasted on the back of prints to make them lay flat.

Prism

A triangular-shaped piece of glass quartz or similar material. A ray of light falling upon one of the sides of a prism is refracted or bent from its original direction at an angle depending upon its own wave-length, the angle of the prism and the material of which the prism is made. Used for the dispersion of light; also in front of a lens in order to reverse a picture as regards right and left.

Prismatic Colors

The colors into which ordinary white light is decomposed by a prism. The order of the colors are, red, orange, yellow, green, blue, indigo and violet. These colors are termed primary or simple colors. If all the colors of the spectrum are recombined white light reappears.

Process Blocks

Half-tone blocks or cuts. A zinc or copper plate on which has been transferred a photographic image. Used for rapid reproduction by being inked in a regular printing press and copies printed on paper.

Lens Protar

(See Lens, Protar.)

Prussiate of Potash

(See Potassium Ferricyanide [red], also Potassium Ferrocya-nide [yellow]).

Pseudoscope

A kind of stereoscope that makes concave parts appear convex, and convex parts appear concave.

Pseudoscopic

The optical phenomena which gives false impressions of visual objects; i. e., distant objects appear near, and vice versa.

Pumice Stone

A form of powder used for polishing metals, cleaning glass, etc., for removing stains from fingers; also for producing a rough surface on negatives for retouching.

Pyrocatechin

C6H6O2 - II, 783.

Colorless crystals. Soluble in water, alcohol, ether, benzine and chloroform. Use. A powerful antiseptic; is a reducing agent and therefore used in various developing formulae.

Pyrogallic Acid

(See Acid Pyrogallic.)

Pyrogallol

(See Acid Pyrogallic.)

Pyro-Metol - II

Pyro-Stains, Removal of.

May be removed from fingers by washing in a 10% solution of oxalic acid, or a 5% solution of sulphuric acid, or a mixture of bleaching lime and citric acid.

Pyroxylin

Nitrocellulose, Gun Cotton, Negative Cotton.

White, flossy substance; possesses physical appearance of ordinary cotton. Soluble in 25 parts of mixture of 1 volume alcohol and 3 volumes ether. Use. When dissolved in equal parts of alcohol and ether it forms collodion, the vehicle used for holding sensitive salts in collodion plates and papers. It is extremely inflammable. Sometimes used to ignite magnesium powder.

Qualitative Analysis

The chemical examination of bodies with a view to ascertain of what substances they are composed.