Degree of enlargement. A term principally employed in telephotography and photo-micrography.

Manganese Sulphate

MnSO4 + 4H2O.

Transparent, pale, rose-red, efflorescent prisms; bitter, astringent taste. Soluble in boiling water; insoluble in alcohol. Used to increase sensitiveness of carbon tissue. Is also an ingredient in ozotype process.


Black or opaque paper having various shaped cut-outs or openings. Used to block out portions of a negative while printing; also, in mounting lantern-slides, the mask being placed between the slide and its cover-glass.


A section of a picture having one tone, and being sufficiently set apart from other portions as to attract a certain amount of attention.


(See Gum Mastic.)


(See Mask.)


A mottled or spotted effect occurring sometimes in silver printing; generally due to the sensitizing bath being too weak.

Melting Point

The degree of temperature at which solids become liquids.

Mercuric Chloride

HgCl2 - II; IV.

Perchloride of Mercury, Bichloride of Mercury, Corrosive

Sublimate, Muriate of Quicksilver. Heavy, colorless, prismatic crystals. Soluble in cold water, I to 16 parts; while in hot water, about I to 4. Soluble in alcohol and ether. In the presence of ammonium chloride it is more soluble, as this chemical converts it into a double chloride of mercury and ammonia. EXTREMELY POISONOUS. Used chiefly for intensification.

Mercurous Chloride

Hg2Cl2. Chloride of Mercury. Impalpable, yellowish-white, heavy powder. Insoluble in water, alcohol and ether. Mercurous chloride is one of the resulting compounds from mercurial intensification, and on the application of ammonia the mercurous chloride is changed to black mercurous ammonium chloride. On the other hand, if sodium sulphite is employed as a blackening agent, the mercurous chloride is reduced to black metallic mercury.




Mobile, heavy, yellowish, lustrous liquid. Soluble in nitric acid, also hot sulphuric acid. Some mercuric salts are extremely poisonous, others very slightly. Used in the Daguerreotype process to develop the image.

Mercury Bichloride

(See Mercuric Chloride.)

Mercury Perchloride

(See Mercuric Chloride.)

Mercury Vapor Lamps

A form of illumination of a peculiar character and highly actinic. Has no red rays in its composition, and for this reason is admirably adapted for artificial portrait work, etc. Obtained by the action of electricity on mercury contained in a vacuum tube.

Metabisulphite of Potash

(See Potassium Metabisulphite.)


A Metric measure of length, equal to 39.37 inches. The standard of this linear measure is one ten-millionth part of the distance between the equator and the north pole.