MEASURING HEAT FROM STARS 433 



^soth 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 con- 

 sider 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 commercializa- 

 tion of everything in and under the heavens. 



II. A Brief Summary of Previous Attempts at Measuring 



Stellar Eadiation" 



The measurement of stellar radiation has been attemptel 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 13.75 inches in diame- 

 ter. 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. 



Eecent measurements of stellar radiation were made by Pfund,* 

 using thermoelements in an evacuated receptacle. The receivers at- 

 tached 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 meas- 

 urements 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 



2 Huggins, Froc. Soy. Soc, 17, p. 309, 1868-9. 



3 Stone, Proc. Boy. Soc, 18, 159, 1869-70. 

 4Pfund, Publ. Allegheny Obs., 3, p. 43, 1913. 



VOL. LXXXVI. — 30. 



