Object Finder

A device used on the mechanical stage of a microscope to mark the position in the field of view.

Object Glass

The lens in a telescope or microscope placed nearest to the object. In the higher-class instruments it is usually composed of two or more lenses cemented together, one of which is flint glass, the remainder being of crown glass.


The front lens or combination of lenses in the telescope, microscope, stereopticon or camera.


Substances of viscous consistency having a more or less unctuous feel; are liquid at ordinary temperatures; are lighter than water, and are insoluble in it, but dissolve in alcohol, and more readily in ether; inflammable when heated in air, and burn with a luminous, smoky flame.

Oil of Lavender

(See Lavender, Oil of.)

Oil of Vitriol

(See Acid Sulphuric.)

Oiled Paper

(See Paper, Oiled.)


The density of a negative, lantern-slide or positive.


A term applied to pictures made by various processes upon opal glass. The glass is coated with emulsion and worked similar to paper. The opal glass is obtainable for photographic purposes with either a plain polished surface or a finely ground surface. Owing to the softer effect given to the picture the latter is usually preferred.


A substance that is impervious to rays of light.

Optical Center

A point on the axis of a lens where every incident ray of light which passes through it continues in a path parallel to its original course; i.e., the direction of the ray is unaltered. The optical center is not necessarily the middle of the lens, or even inside it.

Optical Contact

A print is said to be in optical contact with glass when it is firmly attached to it with no intervening air.

Optical Glass

Usually crown or flint glass, specially manufactured for the construction of lenses.

Optical Lantern

Stereopticon or Magic Lantern. A form of apparatus used for projecting magnified images upon a white screen. Various forms of illumination may be employed.


A term relating or pertaining to vision or sight; subservient to the faculty or function of seeing; i. e., that which assists one to see more clearly.

Organic Chemistry

(See Chemistry, Organic.)

Organic Substance

Pertaining to substances which have organs. Therefore, pertaining to the animal and vegetable life and structure. Ex.: Sugar is an organic product or substance.


A photographic term denoting the correct rendering of colors; i. e., free from the usual photographic fault of exaggerating the deepness of greens, yellows and reds, and the brightness of blues and violets.

Orthochromatic Plates

(See Plates, Orthochromatic.)


To make orthochromatic.