Triumphs and Wonders of the 19th Century: The True Mirror of a Phenomenal Era / A volume of original, entertaining and instructive historic and descriptive writings, showing the many and marvellous achievements which distinguish an hundred years of material, intellectual, social and moral progress

by James P. Boyd

VIII. GRAPHIC CHEMISTRY.

LOUIS JACQUES DAGUERRE.

The honor of discovering that prints could be made by the action of light on certain salts, such as those of silver, belongs to Daguerre, in 1839.

The fundamental principle of graphic chemistry is that metallic salts, sensitive to the light, when in contact with organic matter, suffer a complete or partial reduction and are rendered insoluble. The intensity of the reduction is measured exactly by the intensity of the light. When light is reflected from any object capable of producing different degrees of intensity, as from the hair and face of a man, the reduction of the metal is greatest by the light from that portion of the physiognomy which gives the greatest reflection. Thus, when the unreduced metallic salt is washed out, a permanent record, the negative, of the object is left.

It is a long step from the first daguerreotype to the modern photograph, but the principle of the process has remained unchanged.

Photographs in natural colors have of late years been obtained. One method is by interposing a film of metallic mercury behind the sensitive plate which must be transparent. The reflected rays of light, having different wave lengths, precipitate the metal in superimposed films, corresponding to the wave or half-wave length. When a negative thus formed is seen by reflected light, the emergent rays from the superimposed films acting as mirrors are transformed into the original colors of the photographed object.

The various methods of printing by heliotypes, photolithographs, photogravures, etc., are illustrations of the application of graphic chemistry to the arts.