(Gr. &KTis, ray, p rpov, measure), an instrument for measuring the heating and chemical effects of light. The name was first given by Sir John Herschel to an apparatus for measuring the heating effect of solar rays (Edin. Journ.. Science, 1825); Herschel's instrument has since been discarded in favour of the pyrheliometer (Gr. rvp, fire, i Xeos, sun). (See Radiation.) The word actinometer is now usually applied to instruments for measuring the actinic or chemical effect of luminous rays; their action generally depends upon photochemical changes (see Photo-Chemistry). Certain practical forms are described in the article Photography.