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



Difference per 

 Load. Extensometer. half-ton. Set. 



3 424 



3 468 to 470 44 to 46 



4 528 to 540 58 to 70 



249 49 



4 545 to 550 

 41 670 



715 after half a minute. 



758 after two minutes. 



785 after nine minutes. 



447 247 



41 798 



5 1?00 running slowly off the scale. 



The mean extension from to 2^ tons is 73'6 per ton, which makes 

 E = 13740 tons per square inch. 



II. " The Electrical Measurement of Starlight. Observations 

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

 meath, in April, 1895. Preliminary Report." By G. M. 

 MINCHIN, M.A. Communicated by Professor FITZGERALD, 

 F.R.S. Received April 29, 1895. 



The method employed in these experiments for measuring the 

 intensity of the light which reaches the earth from the stars and 

 planets consists in the determination of the electromotive force gene- 

 rated by such light in certain photo-electric cells, the square of this 

 electromotive force being proportional to the energy of the incident 

 light. 



It will, then, be well to describe first the nature and construction 

 of these cells. 



The Photo-electric Cells. 



In these cells the surface on which the incident light is received is 

 formed by depositing a thin layer of selenium on a surface of clean 

 aluminium, and immersing the sensitive layer in a glass cell filled 

 with cenanthol. 



The mode of formation of the sensitive surface is as follows : 



