780 



Prof. H. Rubens and Mr. H. Hollnagel on 



for potassium bromide 0*216, and for potassium iodide 0'104. 

 The effect of the selective absorption of quartz on the 

 wave-length measurements of the Reststrahlen of rock-salt 

 would be twofold ; firstly, to cause a slight displacement 

 of the wave-length X x and X 2 °£ the maxima in the energy 

 distribution curve of a few tenths /j, toward greater wave- 

 lengths; secondly, to produce a considerable change of the 

 ratio. 



For the determination of the reflectivities the apparatus 

 drawn in fig. 12 was introduced into our arrangement given 



in fig. 2 instead of the interferometer. The source of light 

 A and the concave mirror B having been located, the mirror 

 Si was so adjusted as to give a distinct image on the hori- 

 zontally placed surface under investigation at 0, from where, 

 "upon retlexion via the mirror S 2 , the rays entered K and 

 proceeded along the prescribed path of fig. 2. All surfaces 

 were brought to the same height by the use of indicator Z, 

 and adjusted horizontally by the aid of a sensitive spirit-level. 

 A summary of results obtained for the reflectivities of 

 various substances for the Reststrahlen of potassium 

 bromide follows. The reflexion power of silver is taken as a 

 basis =100. 



Reststrahlen of Potassium Bromide. 



Substance, 



Reflectivity. 



1 Potassium Bromide ... 



Potassium Iodide 



Sylvine 



82*6 per cent. 

 29-6 „ „ 

 36-0 „ „ 

 25-8 „ „ 

 197 „ „ 

 14'0 „ „ 

 9-6 „ „ 





Fluorite 



Glass 



Water (2—19°).... 





