Roll Holder

An attachment usually employed for holding roll films. It consists principally of a wooden box having a spool in each end. The film is unwound from one spool onto another to a sufficient length to allow of an unexposed section being presented for exposure. The front or opening of the holder is covered with a slide, when not in use.

Hydrochloric Acid

(See Acid, Hydrochloric.)

Hydrofluoric Acid

(See Acid Hydrofluoric.)



The lightest of all elements and one of the most important. It is a gas and is taken as the unit for atomic weights. It is one of the elements of water, and the gas in combustion with oxygen gas is used as an illuminant for stereopticons, enlarging lanterns, etc.

Hydrogen Chloride

(See Acid Hydrochloric.)

Hydrogen Dioxide

H2O2. Hydrogen Peroxide.

Colorless, slightly acidulous liquid. An excellent disinfectant. Is a powerful bleaching agent. Used to remove stain from paper and also as a hypo eliminator.

Hydrogen Peroxide

(See Hydrogen Dioxide.)


(See Acid Hydrofluoric.)


An instrument for ascertaining the specific gravity or relative density of liquids for photographic work. The zero point should appear at the surface when the hydrometer is placed in distilled water at a temperature of 58 degrees Fahr. Each degree marked downward indicates a density equal to 1% of common salt.


C6H6O2 - II; IV; V.

Hydrochinone or Hydrokinone; also Quinol. Colorless crystals. One of the most valuable of developing agents. May be used repeatedly. Gives negatives of a blue-black color. Tends to give harshness and, therefore, is useful in copying. Hydroquinon combined with metol makes an excellently balanced developer for general work, as metol tends to produce flatness.

Hypergon Lens

(See Lens, Hypergon.)


An abbreviation for sodium thiosulphate, commonly called sodium hyposulphite. (See Sodium Hyposulphite.)

Hypo Eliminator

A solution or chemical used for the removal of sodium hyposulphite. The process is one which changes the hypo to a less injurious compound and also makes it possible to easily remove the hypo from either plates or papers. In using a hypo eliminator great care must be exercised that the directions be followed implicitly; otherwise even more objectionable compounds than hypo may be formed. Where time is not a controlling factor washing is the best of all hypo eliminators. (See Eau de Javelle.) An excellent hypo eliminator is a saturated solution of alum. The best chemical destroyer of hypo is potassium percarbonate. Rinse the plate after taking it from the fixing bath and then place in a tray half full of water and add 6 grains of potassium percarbonate for every 4x5 plate. Rock the tray. Remove the negative when the liquid ceases to efferversce. Wash for five minutes.