Hyposulphite of Soda

(See Sodium Hyposulphite.)


Water frozen into a solid mass. Water expands when in the act of freezing. Ice is very essential in warm weather for keeping the various photographic solutions cool. Various freezing mixtures can be used as substitutes for ice. (See Ammonium Nitrate.)


The amount of illumination of a surface depends upon the distance of that surface from the light. In fact it varies inversely as the square of this distance. Ex. If a certain quantity of light falls upon a certain area at a distance of 6 inches from the illuminant, the same quantity of light will cover four times that area if the distance be increased to one foot; 9 times at a distance of 18 inches; 16 times at a distance of 2 feet, etc. This same principle is carried out, of course, in contact printing or enlarging, and if an exposure of one second is necessary at a distance of one foot from the source of illumination, 4 seconds will be required at 2 feet, etc.


An optical term denoting the picture thrown by the lens upon the focusing screen or sensitive plate. When the light acts upon the sensitive plate in the camera, it forms an invisible image (also termed latent image). This is rendered visible by the chemical action of the reducing agent, such as a developer. When visible it is called a photographic image.

Double Image

A blurred or double image is generally registered on the plate when the camera is moved during the exposure.

Latent Image

The name given to the invisible image formed by the action of light coming through the lens in the camera and affecting a sensitive photographic plate. The image is rendered visible upon the application of a developer (reducing agent).

Reversal of Image

(a) An effect produced by extreme over-exposure of a sensitive plate and the action of light which produces a positive instead of a negative. (b) An iiuage is also reversed by mears of a prism which is placed in front of the lens. It is, of course, necessary that the object be located at right angles to the direction in which the lens is pointed in order that it may fall properly upon the prism. The image will then be cast upon the lens in a reverse manner than when the prism was not employed; therefore, the resulting image on the sensitive plate will be reversed. The prism is used principally in photo-mechanical work.

Angle of Incidence

The angle formed by a ray of light falling on a flat surface with a line perpendicular to that surface.

Line of Incidence

A Straight line formed by a ray of light moving toward a plane surface.

Index of Refraction

(See Refractive Index.)

India Ink

An ink composed of exceedingly fine lampblack. Sold in sticks and cakes. Used for spotting negatives and prints, also for working up bromides, etc.

India Rubber

Caoutchouc. A compound of hydrogen and carbon. A juice obtained from the India rubber tree. Used in making rubber tubing, squeegees and other photographic accessories. Combined with sulphur forms ebonite, vulcanized rubber, etc., in which form it is used in the manufacture of trays, slides for plate-holders, etc.