1877.] 



Movements of Radiometers. 



551 



a dark band of reversal in its middle, save that, instead of being confined 

 to extremely narrow limits of refrangibility, the central space and its 

 wings would be of wide extent. It follows from the experiments that, in 

 the complete radiation from glass, the portions of the spectrum called the 

 wings together act negatively, the portion between positively. It does 

 not, of course, follow that each wing acts negatively, but only that the 

 balance of the two is negative. When the tumbler is heated a little over 

 212° there is a slight positive action from radiation which passes directly 

 through the bulb. The circumstances lead us to regard this as an exten- 

 sion of the right wing ; for it comes from a depth, measured from the 

 inner surface of the bulb in glass, i. e. not counting the intervening air, 

 somewhat greater than the thickness of the wall of the bulb ; and we 

 know that the more a solid body is heated, the higher, as a rule, does the 

 refrangibility of the radiation which it emits extend, and the greater the 

 proportion of rays of high to those of low refrangibility. It is simplest, 

 therefore, to suppose that the action of the right wing, like that of the 

 space between the wings, is positive, and that the observed negative 

 action in the experiment of § 7 is due to the excess of negative action of 

 the left wing over positive action of the right. In the mica radiometer 

 the experiments indicate no such difference of action in the different 

 layers of the bulb as in the case of the pith radiometer. Hence taking, 

 in accordance with what now appears to be made out to be the theory of 

 the motion of the radiometer, the direction in which the fly is impelled 

 as an indication which is the warmer of the two faces of the disks, and 

 that again as an indication which is the darker with respect to the radia- 

 tion to which it is exposed, we arrive at the following results as regards 

 the order of darkness of the substances for the three regions into which 

 the spectrum of the incident radiation has been supposed to be divided, 

 the name of the lighter substance being in each case placed above that of 

 the darker : — 



T », • Region of intense . , , . 

 Left wmg. abs £ ption by glasg Right wing. 



From pith radiometer { lampblack. £^ pblack 



-,. , f Lampblack. Lampblack. Mica. 



From mica radiometer j Mica f Mica * Lampblack. 



Hence, on descending in refrangibility, the order of darkness of the 

 two substances of either pair is at first the same as for the visible spectrum, 

 and at last the opposite ; and the reversal of the order takes place sooner 

 with mica and lampblack than with pith and lampblack. The order 

 falls in very well with that of the chemical complexity of the three 

 substances. 



20. The whole subject of the behaviour of bodies with respect to 

 radiant heat of the lowest degrees of refrangibility seems to me to need a 

 thorough experimental investigation. The investigation, however, is one 



2q 2 



