102 M. B. Pringsheim on the Radiometer. 



at several vanes, so that any possible opposed actions of two 

 vanes must first be ascertained by special experiments ; while 

 it is very difficult really to determine what portion of the 

 motion proceeds from one vane alone, since this is frequently 

 disturbed by reflections of light, which, especially when the 

 action of bright surfaces blackened at the back, with radiation 

 of the bright side, is to be ascertained, are often apt to reverse 

 the entire motion. Besides, the vanes in their rotation change 

 their position with respect to the source of heat, and there- 

 with the essential conditions of the motion, so quickly, that 

 perhaps dissimilar phenomena, separate in time, occurring at 

 one vane, are completely obliterated and rendered undistin- 

 guishable. And, lastly, a relatively strong force is requisite 

 in order to overcome the resistance presented by friction and 

 set the radiometer rotating ; so that it is a comparatively 

 insensitive apparatus. 



These inconveniences are avoided when, instead of a radio- 

 meter, a so-called torsion-apparatus (like several constructed 

 by Orookes) is employed. In these we have, as a rule, to do 

 (unless the differential actions of several faces are to be 

 observed) with one vane only, which is suspended by a thread 

 and, on receiving the radiation, is deflected a certain angle, 

 till the torsion of the thread counterbalances the effective 

 force. Such an apparatus, but of more simple construction 

 than most of Crookes's, I constructed for myself ; and with 

 it I carried out the experiments particularly described below. 



It consisted of a glass tube r, 435 millim. in 

 length and 22 millim. in diameter, blown out at 

 one end into a spherical bulb, k, of 80 millim. 

 diameter ; into the other end a ground glass 

 stopper fitted air-tight. To a small hook pro- 

 jecting from the stopper a very fine glass thread, 

 /, 420 millim. long, was cemented, to the other 

 extremity of which a small, very thin microscope 

 covering-glass, d, silvered behind, was fastened, 

 to serve as a mirror. To the back of this mirror 

 one extremity of the rectangular disk s, of 24 

 millim. length and 16 millim. width, serving as 

 a vane, was cemented with hot wax, while on the 

 other side the disk was counterpoised by a pin, 

 n. The whole, attached to the glass thread, was 

 slowly lowered through the glass tube ; and then 

 the stopper, previously greased, was inserted, 

 air-tight, so that the mirror hung in the centre 

 of the bulb. 



From the glass tube two lateral branch tubes, t x and 



\=s± 



