Measurements in tlie Extreme Infra-red Spectrum. 779 



metallic absorption of water as 79 /z, from the dispersion of 

 the visible and neighbouring spectral domains, and the value 

 of the dielectric constant of water for waves of infinite 

 length. The strong absorption of Reststrahlen of rock-salt 

 and sylvine indicated a tendency in this direction *. It has, 

 however, been shown that the assumptions in the computation 

 that Drnde made are not fulfilled, and hence the results are 

 invalid |. 



Special care was taken to determine the absorption of 

 quartz, this being of particular interest, as our interferometer 

 contained two plates of quartz, and a strong selective ab- 

 sorption of this material would undoubtedly influence the 

 wave-length determinations. The results are summarized 

 as follows : — 



Quartz perpendicular to the Axis J 







Percentage Absorption A for Reststrahlen. 



Thickness. 





Rock-salt. 



Sylvine. 



Potassium 

 Bromide. 



Potassium 

 Iodide. 



nun. 

 1 0-60 



35-8 



14-8 



12-2 





200 



677 



400 



35-5 



20-0 



3-03 



7(>-9 



54 -y 



40-8 



— 



403 



826 



69-4 



57*6 



320 



- ™ =e q * 



In the computation of the percentage absorption A the 

 reflexion on the two surfaces of the quartz, i. e. about 26 per 

 cent., was always considered. It is obvious that the quartz 

 absorption decreases with increasing wave-length, and that 

 the absorption is only strongly selective in the case of rock- 

 salt rays. If one calculates q the absorption constant according 

 to this formula 



100 



it is found to decrease with increasing thickness for the 

 rock-salt rays from 0'70 to 0*47. On the other hand, for 

 the other rays it is approximately constant; for sylvine 0*281, 



* II. Rubens and E. Aschkinass, he. tit. p. 252. 



t H. Rubens and E. Ladenberg, Site. Berichte d. Kon. Preiiss. Wis*. 

 Akad. p. 274 (1908 j. 



| As the quartz plates are cut at right angles to the axis, the direction 

 of the rays is therefore that of the optic axis. 



