7#2 Measurements in the Extreme Infra-red Spectrum. 



potassium iodide have been determined by use of a quartz 

 interferometer. 



The various Reststrahlen, excepting those of potassium 

 iodide, consist of two well-defined bands ; the potassium 

 iodide rays having but one band apparently. 



The different wave-lengths, X 1 of the more intense stripe, 

 X 2 of the weaker, and X the mean wave-length value, 

 together with the molecular weights, are summarized in the 

 following table : — 



Reststrahlen. 



K 



X 2 . 



K 



M. 



Rock-salt 



53' Q [x 



46-9 fi 

 70-3 fi 



51*7 ^ 

 63-4 fi 



58 5 



Sylvine 



.. 62-0 /* 



746 



Potassium Bromide 



... 86*5 /i 



75-6 fi 



82-3 ft 



119-0 



Potassium Iodide 



— 



— 



964ft 



1600 



At a glance one sees a relation between the molecular 

 weight and the mean wave-length A , in lact the latter 

 increases more rapidly than the square-root of molecular 

 weight and less rapidly than the molecular weight itself. 



The index of refraction of water for X=82'3 [x is of the 

 same order of magnitude as in the visible spectrum. 



By the investigation of the Reststrahlen of potassium 

 bromide and potassium iodide our knowledge of the optical 

 spectrum has been extended £ of an octave. Its complete 

 extent is now ten octaves, two of which are in the ultra-violet, 

 one in the visible, and seven in the infra-red. 



By the mean wave-length A we designate the value obtained 

 from the interference curves, when we consider the distances 

 of the neighbouring maxima and minima only at those parts 

 of the curve where they are most prominent. As mentioned 

 above, values thus calculated correspond to the mean wave- 

 length of the whole radiation. 



