ACTINOMETER, an instrument for measuring the heating and chemical effects of light. It is derived from the Greek, &Kris ray, Orpov measure. The name was first given by Sir John Her schel to an apparatus for measuring the heating effect of solar rays (Edin. fours. Science, 1825) ; Herschel's instrument has since been discarded in favour of the pyrheliometer (Gr. 71-0p fire, Oros sun). (See RADIATION.) The word actinometer is now usually applied to instruments for measuring the actinic or chemical ef fect of luminous rays; their action generally depends upon photo chemical changes. A type of actinometer is used in photography under the name of exposure-meter. In this instrument a section of sensitized bromide is exposed for a measured period of time. Certain practical forms are described in the article PHOTOGRAPHY.