Measurements in the Extreme Infra-red Spectrum. 7 



ID 



Reststralden of Potassium Iodide. 



The radiomicrometer gave a deflexion of 5 mm. for the 

 Reststrahlen of potassium iodide ; of this 20 per cent, 

 was produced by short-waved radiation. When the radiant 

 beam passed through the interferometer this deflexion was 

 diminished to 2*8 mm., of which 2*4 mm. were pure Rest- 

 strahlen. In spite of the limited energy it was possible to 

 determine their wave-length as well as the energy distri- 

 bution. To this end five series of observations were carried 

 out ; a fairly good agreement existing for the various series. 

 Three of these are presented in fig. 6 ; ft being observed with 

 the wedge-shaped quartz plates. The series y was observed 

 with a radiomicrometer under a vacuum-tight brass hood, 

 provided with a quartz window 0*5 mm. in thickness. The 

 zero-point under these conditions was so free from disturbance 

 that it was possible to observe seven maxima and minima 

 very clearly. The series was carried a considerable distance 

 further, however, than is indicated in fig. 6, but no maxima 

 and minima could be recorded with certainty. The conclusion 

 to be drawn therefrom is that the energy distribution in this 

 instance has but one intense well pronounced maximum. 



The positions of the maxima and minima for the three 

 series here presented are obtainable from these tables. 



Series a. Series ft. 



Maxima. 



Minima. 



a 



b 



c 



d 



e 



415 



320 



22-8 



140 



50 



a' 



b' 



c' 



cV 



c' 



378 



28-5 



189 



97 



Maxima. 



Minima. 



a 



c 



d 



e 



/ 



41-8 

 32-9 

 23-6 

 Uo 

 5-6 

 90-1 



a' 



b' 



c' 



(V 



e' 



r 



37-8 

 27 9 

 193 

 100 

 0-7 



Series 7. 



Maxima. 



Minima. 



b 



c 



d 



e 



/ 



9 



422 

 23-0 

 23-6 

 14-6 

 5-6 

 96-4 

 S7-3 



a 



b' 



c' 



d' 



/' 



• 



38-0 

 27-5 

 18-6 

 9-5 

 0-7 

 91-3 

 828 



