actinometer

[Gr. ἀκτίς ( aktis ), gen it. ἀκτῖνος (aktinos) = a ray, and μέτρον (metron) a measure. Lit. Measurer of solar rays.] An instrument devised by Sir John Herschel for measuring the intensity of the solar rays. It consists of a thermometer with a large bulb filled with a dark-blue fluid, and enclosed in a box, the sides of which are blackened, and which is covered with gla . It is placed for a minute in the shade, then a minute in the sun, and then one more again in the shade. The mean of the two variations in the shade is then subtracted from that in the sun, and the result measures the influence due to the solar rays. "By direct measurement with the actinometer I find that out of 1,000 calorific solar rays, 816 penetrate a sheet of plate gla 0-12 inch thick; and that of 1,000 rays which have pa ed through one such plate, 850 are capable of pa ing through another."Note in Herschel's "Astronomy,' ono my , 5th ed. (1858), § 396, ăc-tin-om-ět'-ric, a. [ACTINOMETER.] Pertaining or belonging to an actinometer.