Light rays, especially the violet and ultraviolet parts of the spectrum, that produce chemical changes, as in photography.

Actinic Rays, rays capable of producing chemical decomposition, as in photography, in the coloring of flowers and fruit. All ether waves, from all sources and of all lengths, may now be considered actinic, some substances being decomposed by certain wave lengths and other substances by different wave lengths.

Entry from Everybody's Cyclopedia, 1912.

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