Man's physical universe

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THE NATURE OF MOLECULES 583

Photography Is an Application of the Activation of Atoms by Radiant

Energy.

The development of photography has been of inestimable value to

Science and modern life. For example, the scientist has used photography

to study the internal structure of the atoms, the nature of fardistant

nebulae, flaws in metals, and in the diagnosis of disease by use

of X rays. Photography has made possible our modern illustrated

books, newspapers, and magazines, and the motion-picture industry.

It is only necessary to compare silent motion pictures made twenty

years ago with the modern sound motion pictures in color to realize

the tremendous progress that has been made in photography during

the past generation.

It was known for a long time (/. H. Schulze, 1727) that light would

blacken silver nitrate or silver chloride, but for practical purposes

early photography had two great drawbacks: first, no means of fixing

the images were known; second, time required for the exposures was

quite long. Louis Jacques Daguerre, in 1839, made public the details

of his daguerreotype process of sensitizing a silver plate with iodine and

developing with mercury vapor. Fox Talbot (1835) found that if he

treated paper with successive washings of a solution of common salt

(sodium chloride) and a solution of silver nitrate and exposed the

wet paper, he could obtain a much more rapid blackening in light

than with either silver nitrate or silver chloride alone.

Unfortunately the silver chloride left on the paper unchanged by the

light will soon darken unless removed, and such a picture will soon

become black all over when exposed to light.

Later it was found that a silver-salt emulsion could be exposed to

light without producing any visible change but that the portion of the

salt thus activated, even after a period of thirty or more years, would

be reduced by certain substances which develop this latent image and

are therefore called developers; the portions of the emulsion not activated

are not appreciably reduced during the short period of time

required to reduce the activated portion, but continued exposure to

the developer would eventually reduce nearly all of the silver salts in

the emulsion. Fox Talbot discovered the use of a developer (gallic acid)

and reported it to the Royal Society in 1841. The most common

developers are pyrogallic acid, hydroquinone, and methyl-para-aminophenol

sulfate, otherwise known as "Pictol," "Metol," or "Elon."

It was found that larger crystalline particles were more readily

activated than the smaller particles and that the sensitiveness to light

could also be controlled by the use of the bromide and iodide of silver,

as well as the chloride. Silver bromide is more sensitive to light than

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