170 

 3. 



Mr. T. Royds on the Reflective 



The Reststrahlen from Fluorspar. The light from a 

 slit illuminated by an incandescent mantle was 

 reflected at three fluorspar surfaces, the second of 

 which was concave. The maximum of these rays lies 

 at 25-5 p*. 

 4. The Reststrahlen from Rock-salt. The rock-salt plates 

 were arranged exactly as those of selenite. A more 

 intense source, a Nernst filament, was used. The 

 rays were very impure, but a fourth reflexion 

 rendered the intensity too small. In order to increase 

 the purity a clear rock-salt shutter (4 mm. thick), as 

 applied by Rubens, was used instead of a metal one.. 

 The maximum of the pure Reststrahlen lies at 51'2 /jl f;, 

 in our case, however, somewhat towards smaller 

 wave-lengths. 



The following simple observations serve to indicate roughly 

 the purity of the Reststrahlen employed : — 



• 



Deflexions for Reststrahlen 

 from 



Selenite. 



Fluorspar. 



Rocksalt. 



Total energy 



1330 

 3 

 7 



1150 



688 

 

 4 

 



100 

 45 

 62 

 65 



Energy transmitted through glass (3 mm.) 



„ ,, „ quartz (2'5 mm.)... 

 ,, ,, „ rocksalt (2-35 cm.). 



Several corrections are to be applied to the radiation which 

 falls on the thermopile when placed to receive the radiation 

 reflected from the black surface. 



Firstly, diffuse radiation from the incident image may fall 

 on the thermopile. To determine its amount the slit image 

 was placed in the centre of the hemisphere; and the energy 

 falling on the thermopile when placed at a distance equal to 

 that in the experiments (2*5 mm.) was measured. The curve 

 of the deflexions so obtained for all distances of the thermo- 

 pile is shown by the dotted line in fig. 2. It was found that 

 0'42 per cent, of the energy in the incident image fell on the 

 thermopile. 



Secondly, energy is radiated from the black surface since 

 it becomes heated when exposed to the incident radiation. 



* Rubens, Phys. Z.S. iv. p. 726 (1903). 



t Rubens & Asclikinass, Wied. Ann. lxiv. p. 241 (1898). 



