Antidote

II. A substance which has the effect of counteracting poison

Aperture

The size of the opening in the lens through which light is allowed to pass into the camera.

Aplanatic

(See Lens, Aplanatic.)

Apochromatic

(See Lens, Apochromatic.)

Apothecaries' Weight

(See Weight, Apothecaries')

Aqua Regia

Nitro-hydrochloric Acid. A mixture of one part nitric and two parts hydrochloric acids. Gold, platinum and many metallic compounds which do not dissolve in nitric or hydrochloric acid separately are readily soluble in Aqua Regia.

Arrowroot

A form of starch. Chief constituent of several mountants. Used in preparing or sizing papers for various printing processes.

Artificial Light

A term applied to all forms of light other than daylight.

Asphaltum

Asphalt; Bitumen; Mineral Pitch; Judean Pitch.

A brownish-black, resinous mineral. Soluble in benzine, chloroform, or in turpentine. Used by Niepce as a coating for silver plates; employed in various varnishes and lacquers; cell-making for microscopy; etching, and various photomechanical processes.

Astigmatism

A defect of the pencils of light passing through the margin of a lens that renders vertical and horizontal lines unequal and not sharp; usually found in lenses corrected for flatness of field. The defect may be overcome by using a small stop. In a majority of modern lenses this defect has been done away with by the use of Jena glass and proper construction.

Atom

An atom is the smallest particle into which an element can be divided.

Atomic Weight

(See Weight, Atomic.)

Autochrome Plates

VIII. (See Lumiere Process.)

Automatic Shutter

(See Shutter, Automatic.)

Autotype

A. trade name for the carbon process.

Avoirdupois Weight

(See Weight, Avoirdupois.)

Axis

The axis of a lens is an imaginary line passing through the center of its curve and at right-angles with its plane surface.

Principal Axis

The principal axis of a lens is a line passing through the center of curvature of spherical surfaces and perpendicular to the plane surface; or, it may be described as a straight line passing through the optical center of a lens.

Back Focus

(See Focus, Back.)

Repeating Back

An attachment on the back of a camera for making a number of different exposures on one plate.

Reversible Back

A removable attachment at the rear of the camera, enabling one to insert the plate-holder either in an upright or horizontal position.

Revolving Back

An improvement on the reversing back in which the rear attachment may be changed from vertical to horizontal, or vice versa, without the necessity of removal.

Swing Back

1,84, 85; III.

An arrangement by which the back of the camera may be inclined so as to place the focusing screen or plate-holder in a true vertical position when the camera is tilted. This is to avoid the distortion of lines. A SINGLE SWING BACK is one in which the swing is confined to one plane, while a DOUBLE SWING BACK is one in which the back is arranged to move upon both a horizontal and a vertical axis.