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silver chloride. A mixture of the bromide and iodide (within certain

limits) is more sensitive than either the pure bromide or iodide.

Photographic films which are very sensitive to light are called fast

films because they can be used with a fast shutter speed (short exposure).

A slow film, on the other hand, requires long exposures. Suspensions

of these salts in specially prepared gelatine may be made

extremely sensitive to infrared, ultraviolet, and other portions of the

electromagnetic spectrum by the addition of special sensitizing agents

and by heat treatment of the emulsion. One of the first observations

which led to the modern sensitized films was based on the fact that

films whose emulsion contained gelatine made from cows which had

been eating plants containing mustard oil were unusually sensitive.

This observation led to the use of mustard oil (allyl isothiocyanate)

to sensitize emulsions.

The emulsion may be coated on celluloid or

cellulose nitrate or acetate film bases, glass plates, or paper.

After exposure to light, the film or plate is placed in the developing

bath, which contains one or more of a number of substances which will

reduce the exposed areas, liberating free silver from its salts.

The size

of the silver particles and other factors determine the grain, which is

very important in enlarging. Developers which produce very small

free silver particles are called fine-grain developers. The speed and

contrast in developing are controlled by the use of substances such as

sodium carbonate (the accelerator), to provide an alkaline solution,

and potassium bromide (the restrainer), which helps to prevent reduction

of the unactivated silver salts. Sodium sulfite (the preservative)

is also added to the developing solution to prevent deterioration due

to the action of the reducing agent with the oxygen of the air.

The unexposed compounds remain unchanged after the reduction

of the exposed portion. These unchanged compounds would also be

reduced if left exposed to the light, so they are treated with "hypo"

(sodium thiosulfate) in the fixing bath, which dissolves these unreduced

silver salts. An acid is generally added to the fixing bath to stop the

action of the developer because it neutralizes the alkali which is essential

to the action of the developer. Potassium or chrome alum is added

to harden the gelatine of the emulsion, which is softened and swollen

by the sodium carbonate or other alkaline substance in the developing

solution. Sodium sulfite is also added to oppose the decomposition of

the "hypo."

Ordinary films and plates when developed are called negatives.

Negatives represent the reverse of the light values of the original subject.

By passing light through a negative held in contact with a

paper, film, or plate coated with photographic emulsion, the tone

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