158 



SOME RECENT ASTRONOMICAL EVENTS. 



Notwithstanding- this discouraging evidence, the question was again 

 taken up within the last two or three years by Professor Nichols with 

 his radiometer. Before mentioning his results it will be of interest 



to briefly describe that instrument. The 

 principle upon which it is based is the 

 well-known one of the Crookes revolving 

 vanes, familiar in the collections of appa- 

 ratus exhibited in the physical cabinets of 

 academies and colleges. In this interest- 

 ing toy a pair of small metallic vanes, 

 blackened on opposite sides and tixed 

 perpendicularly upon a light arm, itself 

 horizontal and delicately poised at its 

 center upon a vertical axis, is caused 

 to rotate in a vacuum by the influence of 

 light. 



The Nichols radiometer is merely this 



instrument adapted to measure the 



itensity of the impinging rays. An idea 



O-L-© , 



[Ff? 



The Nichols radiometer. From Astrophysical Journal, Vol. xiii, No. 2, March, 1901. 



of its construction is given by the accompanying diagram. The vanes, 

 made very small, are fastened at the ends of a slight stem of glass about 

 one-fourth of an inch long, which in turn is tixed at right angles to a 

 second longer glass stem furnished with a very light mirror and sus- 



