M. E. Pringsheim on the Radiometer. 117 



On irradiation with sunlight, a positive deflection, with the 

 irradiated side receding, occurred (the reverse of what took 

 place with the mica). This phenomenon distinctly shows that, 

 in spite of the good conducting-power and the thinness of the 

 vane, absorption produced a lasting difference of temperatures 

 between the front and the back side. That any difference 

 which may have existed in the shape of the two sides was not 

 the cause of the deflection was evident from the fact that 

 irradiation of the other side of the vane had for its result a 

 deflection in the contrary direction. The magnitude of this 

 deflection amounted to about 50 divisions of the scale. A 

 one-sided lateral irradiation of the glass case by sunlight pro- 

 duced, as might have been anticipated, no deflection, just as 

 w T hen the thin mica plate was employed. 



When a bright gas-flame was employed as the source of 

 heat a somewhat weaker deflection appeared than with sun- 

 light, and which also had a quite different character. 



While, namely, the sun produced a sudden deflection, here 

 the deflection took place quite slowly, and thereby showed 

 that its cause was a quite different one. In fact it appears to 

 me very probable that in the copper foil only a very slight 

 difference of temperature arises from the absorption of the 

 rays emanating from the gas-flame, and that the greater part 

 of the deflection is produced by the absorption in the glass 

 case. An argument in favour of this is the fact that, with 

 equal distance of the source of heat, the deflection was almost 

 exactly the same on lateral irradiation of one half of the glass 

 case as on full irradiation of the apparatus from the front. 

 There resulted, namely, when the gas-flame was at 305 mil- 

 lim. distance from the vane, a deflection of the index, 



On lateral irradiation, of . . . 23*5 millim. 

 „ irradiation from the front, of 23'0 „ 



The force produced by direct absorption in copper foil ap- 

 pears therefore to be almost exactly equal to that generated 

 by the heating of the back part of the glass case. 



Just as with the mica vane, here also by smoking the out- 

 side of the hinder half of the glass case the direction of the 

 deflection was reversed — with sunlight directly, with the gas- 

 flame by employing glass plates. Thereupon the Dutch- 

 metal lamina was smoked on one side over an oil-of-turpentine 

 flame and again put into the apparatus. 



Irradiation of the bright side with sunlight gave a very 

 strong positive deflection, in which the bright side receded 

 until the smoked side began to come within the range of the 

 radiation, whereupon a violent oscillation resulted. 



