Note:  Do not rely on this information. It is very old.


Actinism, that property of certain kinds of light which produces chemical action, as distinct from their heating or light-giving powers. Thus, of the constituent rays of the sun's light the actinic rays are those at and beyond the violet end of the visible spectrum, those rays at the other end producing no apparent chemical effect. If the actinic rays be screened off by a piece of ruby-glass, which prevents the passage of any rays but the red, no chemical effects will be produced. Hence the use of ruby-glass lanterns in photography.