434 THE POPULAR SCIENCE MONTHLY 



Vega: Altair = ?>:'7, which is at variance with the results obtained on 

 Mt. Hamilton, and emphasizes the importance of making observations 

 through an atmosphere free from water vapor. He concluded that with 

 a more sensitive galvanometer and one of the largest reflectors it would 

 be possible to observe stars to the fourth magnitude. 



An extensive series of measurements of the radiation from ArcturiLs, 

 Vega, Jupiter and Saturn were made by Nichols^ by means of his ra- 

 diometer, which, like the thermopile, absorbs all the radiations of all 

 wave-lengths falling upon it. The receivers were 2 mm. in diameter. 

 A candle at a distance of 1 m. would have given a deflection of 724 

 mm. He used a two-foot reflector and observed deflections of 1 to 2 

 mm. The sensitivity of his radiometer was such that a deflection of 

 1 mm. would be caused by 1/68,750,000 of the heat received on a surface 

 equal to the aperture of the concave mirror from a candle at 1 meter 

 distant. Or, neglecting atmospheric absorption, the sensitivity was 

 such that by using the two-foot mirror to focus an image of the flame 

 upon the radiometer, he would have obtained a deflection of 1 mm. 

 from the candle placed at a distance of 5 miles. He concluded that the 

 thermal intensity was Vega: Arctwus: Jupiter: Saturn =: 1:2.2 :4:.7 : 

 0.74. As for the possibility of further work he concluded that by using 

 a five-foot reflector it would be possible to observe white stars down 

 to the second magnitude and red stars possibly to the third magnitude. 



The Boys^ radiomicrometer has also been tried in measuring radia- 

 tion from stars. The instrument was used with a sixteen-inch reflecting 

 telescope. The slight deflections obtained on various planets and stars 

 were regarded as of questionable origin. 



The earliest measurements of the light from stars by means of a 

 selenium cell were made by Minchin,'^ who used a 2-foot reflector. He 

 examined about a dozen stars, some being as small as the third magni- 

 tude. Owing to the peculiar properties of the selenium cell, which is 

 highly selective in its response to radiations of different wave-lengths, 

 the data can not be used in comparing the radiation from different 

 stars. The seleniiun cell can be applied, however, in the measure- 

 ment of the maximum and minimum of light emission from a variable 

 star which does not change in color. For this purpose it has been used 

 by Stebbins^ in connection with a 12-inch refractor. 



III. A Beief Account of the Present Measurements of 



Stellar Radiation. 



The telescope used in the present investigation was the well known 

 Crossley Eeflector which is part of the equipment of the Lick Observa- 



5 Nichols, Astrophys. Jr., 13, p. 101, 1901. 



6 Boys, Proc. Roy. Soc, 47, p. 480, 1890. 



TMinchin, Proc. Boy. Soc, 58, p. 142, 1895; 59, p. 231, 1896. 

 sStebbins, Astrophys. Jour., 32, 185, 1910 j' 33, 385, 1911. 



