1895.] The Electrical Measurement of Starlight. 153 



Comparison with the Photometric Method. 



In the ordinary method of comparison of " magnitudes," if B and 

 B' are the brightnesses of two stars whose magnitudes are m and m', 

 respectively, we have, by definition, 



p 



log , = T *o (m'-m) ................ (5). 



Now, taking Arcturus and Begulus as of the magnitudes 0'3 and 

 m', respectively, and the electromotive forces of their lights as 8 and 

 4*75 (determination of April 11, previously cited), we have 



.'. m' = 1-43. 



The magnitude of Begulus, as a matter of fact, is variously cited 

 as from 1'42 to 1'7 ; thus the amount of correspondence between the 

 photo-electric and photometric methods is seen. 



Concluding Bemarks. 



Among the few bright stars which we found available was Procyon, 

 and even this star offered an opportunity for observation during a 

 very limited time, owing to mechanical hindrances in the Observatory. 

 The stars in the Great Bear shone brilliantly, and, under favourable 

 circumstances, their light could have been easily measured. The 

 constellation was, however, so nearly vertical, that the aperture in 

 the roof of the Observatory was not sufficiently wide to suit the 

 aperture of the telescope, and hence no observation of any of these 

 stars was attempted. 



On one night observations of Procyon and Regulus were taken. The 

 readings were much smaller than had been anticipated from the great 

 sensitiveness of the cell and electrometer. When these observations 

 were completed, the cell was exposed to a candle at 9 feet, and the 

 effect was so small, that it was evident that some accidental circum- 

 stance was intervening. The cell was, therefore, taken down from 

 the telescope and examined, with the result that we found an opaque 

 movable portion of the cell holder covering a portion of the sensitive 

 surface in the cell. This was at once removed, and then the candle, 

 Arcturus, and Vega gave the large deflections quoted in the observa- 

 tions of the 15th. It was, however, then too late to get Procyon 

 again. But the observations which had been made with this star 

 and Regulus, while the partial obstruction of the cell remained, gave 

 the mean of their deflections as 



Regulus .......... 1*27 mm. 



Procyon .......... 1'90 



