146 Mr. G. M. Minchin. [May 16, 



proportional to the square of the electromotive force generated. If 

 one candle held at a certain distance from the cell gives a difference, 

 E, of potential between the poles P, P', two candles close together 

 will be found to give a difference Ev 2. Or, if a candle be tried at 

 different distances from the cell, the difference of potential will be 

 found to vary inversely as the distance. 



Intrinsic Energies of Stars. 



If I is the total amount of energy radiated into space in any time 

 by a star at the distance r from the earth, the quantity received on 

 any given surface on the earth will be proportional to I/r 2 ; and if E 

 is the electromotive force which this generates in a given cell, we 

 have 



I/r a = fcE 3 (1), 



where k is some constant. Hence, if I' is the intrinsic energy of 

 another star at the distance r', and E' the corresponding E.M.F., 



I'/r' 2 = fcE' 2 (2), 



from which we have 



\, = ^ (3). 



Hence, if the parallaxes of the two stars are known, say p and p' 

 respectively, we have 



When it is desired to compare the energy of a star with that of the 

 sun, we must know the area of the sensitive layer, A, Pig. 1, of sele- 

 nium in the cell. Let this be a, and let A be the area of the aperture 

 of the telescope. 



Then, since it is not desirable to concentrate on the selenium the 

 amount of solar light which falls on the large area A, we must turn 

 the cell to the sun without the aid of the telescope. Let E be the 

 E.M.F. observed, S the intrinsic energy of the sun, and r the distance 

 of the sun from the earth. Then 



a^ = JcW (5), 



while, for any star whose distance is R, giving an E.M.F. equal 

 to e, 



^1 = ^ (6). 



/JL.A/- 



S AVE 



