Page:Popular Science Monthly Volume 86.djvu/437

Rh $1/480$th as bright. This is not saying that previous attempts were of no avail; for they were the stepping stones which aided in what small success was attained with the Crossley reflector on Mt. Hamilton, this past August. If, then, the "layman" with his question of the practical side will please wait a little while longer, and in the meantime consider that the present work is simply another stepping stone in the path of conquest of the secrets held in the firmament, it will greatly help the investigators who are not concerned with the immediate commercialization of everything in and under the heavens.

The measurement of stellar radiation has been attempted by three methods: (1) by means of thermoelements, (2) by means of a Nichols radiometer, and (3) by means of a selenium cell.

Among the earliest attempts by means of thermoelements are the measurements of Huggins. He used one or two pairs of elements of bismuth-antimony in the focus of a refractor having an aperture eight inches in diameter. He recorded positive deflections for Sirius, Pollux, Regulus, and Arcturus. The data given are very meager. It required from four to five minutes (fifteen minutes in one record) to obtain a reading.

Thermoelectric measurements of the radiation from Arcturus and Vega were made by Stone who used a refractor 12.75 inches in diameter. In spite of the excessively long time (about ten minutes) required to obtain a reading he appears to have obtained fairly reliable results. His measurements show that Arcturus emits more radiation than does Vega; his numerical measurements for June 25, 1869, being Arcturus: Vega$$=$$3:2. Considering the fact that the infrared radiations from Arcturus suffer greater absorption than those of Vega in passing through an air mass highly saturated with water vapor, and in passing through the glass lenses of the refractor this ratio (3/2) is in close agreement with subsequent measurements using a reflecting telescope.

Recent measurements of stellar radiation were made by Pfund, using thermoelements in an evacuated receptacle. The receivers attached to the junctions of the bismuth alloys (BiSn—BiSb) were about 1.2 mm. in diameter. The sensitivity was such that the radiation from a Hefner lamp at a distance of 1 m. gave a deflection of 2400 mm. He used a reflecting telescope thirty inches in diameter, and made measurements on Vega (7.5 mm. deflection), Jupiter (part of disk; 3 mm.), and Altair (2.0 mm. deflection, sky hazy). The ratio of the radiations