3 896.] 



The Electrical Measurement of Starlight. 



231 



If. " The Electrical Measurement of Starlight. Observations 

 made at the Observatory of Daramona House, co. West- 

 meath, in January, 1896. Second Report." By G. M. 

 Minchin, M.A., F.R.S., Professor of Mathematics, Cooper's 

 Hill College. Received January 23, 1896. 



In the 1 Proceedings of the Royal Society,' vol. 58, pp. 142, &c, will 

 be found the first account of the measurement of stellar radiation by 

 means of the electromotive force generated by the action of star- 

 light on my photo-electric cells. The observations were resumed in 

 the beginning of January, 1896, in Mr. W. E. Wilson's observatory 

 by the same three observers — namely, Mr. Wilson, Professor Fitz- 

 gerald, and myself. 



Our appliances were substantially the same as before; but the 

 photo-electric cells had in the meantime been rendered markedly 

 more sensitive by the prevention of the ascent of the liquid 

 (cenanthol) into the glass tube containing the aluminium wire, the 

 end of which was coated with the selenium layer on which the light 

 was received. 



This ascent of the liquid was prevented by simply painting the 

 circle of contact of the wire and tube with liquid gelatine, and 

 allowing the gelatine ring thus formed to dry. As gelatine is not 

 dissolved by cenanthol, the liquid was in this way kept out of contact 

 with the aluminium wire, and the result was a great increase in the 

 sensitiveness and lasting power of the cell. 



It is to be regretted that out of the fourteen nights at our disposal 

 for work, only one proved favourable for observations ; but even on 

 this night we were obliged to omit observations on such bright stars 

 as Sirius, Rigel, and Capella, owing to the persistence of haze in the 

 atmosphere. 



The aim of our work on this occasion was to measure the candle 

 power of the light of each of the stars in Orion and the Great Bear, 

 together with that of as many others as spare time would allow us 

 to observe. 



As one cell may not be exactly the same in sensitiveness as 

 another, I have set down in the following table the value of the light 

 of each star in terms of that of a standard candle held at a distance 

 of 10 ft. from the sensitive surface in the cell ; but the value of the 

 light in volts is easily deduced from the data. 



The electrometer employed was that previously used and de- 

 scribed ; and it continued to give with remarkable constancy a 

 deflection of 630 mm. on a scale distant 7 ft. from the mirror for one 

 Leclanche cell, or, say, 432 mm. for 1 volt. 



