[f. ACTINO- + μέτρον measure.] An instrument for measuring the intensity of the sun’s heating rays; first invented by Sir J. Herschel, and described in Edinb. Journal of Science for 1825.
1833. Sir J. Herschel, Brit. Assoc. Report, 379. The actinometer is an instrument … for measuring at any instant the direct heating power of the solar rays.
1879. Photogr., in Cassell’s Techn. Educ., III. 326. The consequent progress of the printing may be most accurately determined by means of the actinometer.
1880. Radcliffe, in Contemp. Rev., Feb., 210. In interplanetary space, if the experiments with the actinometer are to be trusted, the temperature is not less than 256° Fahrenheit below the freezing point of fresh water.
