Measurements in the Extreme Infra-red Spectrum. 777 



our interference curves, by a consideration of the distance of 

 only those maxima and minima approximately in phase, are 

 51*7 fi and 63'4 fi 9 a very good agreement for the rock-salt 

 rays, and only an approximate one for those of sylvine. 

 However, a better agreement cannot be expected, since the 

 meaji wave-length in the grating determination and the mean 

 value in the interferometer measurements are not defined 

 alike. 



Absorption and Reflexion of the Reststrahlen of 

 Potassium Bromide. 

 The properties of rock-salt and sylvine rays are well 

 known ; on the other hand, Reststrahlen of potassium 

 bromide and potassium iodide belong to a portion of the 

 spectrum of which nothing has been known to the present. 

 It seemed or.' interest therefore to study the properties of 

 these rays in various directions. Measurements were 

 principally carried out with the Reststrahlen of potassium 

 bromide, those of potassium iodide being too weak for most 

 purposes. Though the amount of energy in the latter case 

 was so limited, still the absorption o£ various substances for these 

 waves of almost ^ mm. length was determined. 

 The various transmission data are given in the following table; 



1 





Percentage Transmission D. 



Substance. Thickness. 



Reststrahlen of 

 Potassium Bromide. 



Reststrahlen of 

 Potassium Iodide. 



Quartz 



mm. 

 060 



200 



303 



4 03 



290 



002 



040 



00026 



3-48 



300 



2-20 



410 

 40000 

 40000 



649 

 476 

 39-2 

 31-4 

 476 

 517 

 303 

 77-3 



















38-7 



1000 



592 



504 

 54-5 

 250 

 430 





 

 

 

 330 









Paraffin 



Mica 



Ebonite 



Water 



Fluorite 



Rock-salt 



Glass 



Sylvine 



H 2 vapour 



co 8 







