232 The Electrical Measurement of Starlight. [Jan. 30, 



Observations made in January, 1896. 



Source of light. 



Deflection on 



scale, in 

 millimetres. 



"Value cf light, 



in terms of 

 standard candle. 





18 -70 



— 





61 -20 



3-272 





12 -80 



0-635 





3-17 



0-170 





3'29 



0-175 





5-21 



0-279 





5 07 



-271 





4-87 



0-261 





4-90 



0-262 





310 



0-166 





2-20 



117 





2'44 



0*130 



One observation was attempted for Rigel ; but when a deflection 

 of 8 mm. had been attained, the light was cut off by clouds, and no 

 further opportunity occurred. 



With regard to the last column in this table, be it understood that 

 it gives the ratio of the JE.M.F. 'produced by the light of the star as con- 

 centrated in a telescope of 2 ft. aperture to the JS.M.F. produced by the 

 light of the candle falling directly on the cell from a distance of 10 ft. 



In order to determine the candle value of any star, we must, of 

 course, take into account the area of the telescope aperture, the area 

 of the sensitive selenium surface in the cell, the distance of the star 

 from the earth, and the distance of the candle from the cell. 



Thus, if I = intrinsic brightness of a star (i.e., the total amount 

 of energy radiated in all directions by the star per unit time) 



i = intrinsic brightness of candle, 

 A = diameter of aperture of telescope, 



a = diameter of selenium surface, 

 R = distance of star, 



r = distance of candle, 



e = deflection on electrometer scale dne to star, 

 E = deflection on electrometer scale due to candle, 



we have 



I _ /a.B,.e\ 2 

 i~\A.r.E] ' 



For example, selecting the star Procyon in the above list as that 

 whose parallax is best known, we find (see Miss Clerke's ' System of 

 the Stars ') the value 266 second for the parallax. Also the dia- 



