134 INFRA-RED ABSORPTION SPECTRA. 



lengths at 3.43 fi, and everything ever done with the infra-red absorption 

 spectra that is worth considering is in agreement as far as the absolute 

 value of wave-lengths is concerned. 



In the same manner the absorption band at 13.8 to 13.9 ju., which is a 

 close harmonic with the one at 3.43 ju and 6.86/* (true value= 13.72 /x), 

 indicates the possibility of the observed indices^ of rock salt at 13 ju, 

 being somewhat too low. The computed values are higher. Of course, 

 it may ultimately be found that this apparent harmonic series is really 

 a slowly converging " spectral series," in which case there ought to be 

 this discrepancy between the computed value, 13.72 /x, and the observed 

 values, viz, 13.8 to 13.9 {x — the latter values being more nearly correct. 



The whole shows that the absolute value of the wave-lengths of the 

 absorption bands are known as accurately as is possible with the present 

 knowledge of the dispersion of rock salt. 



iRubens & Trowbridge : Ann. der Physik, 60, p. 733, 1897 ; corrections in Ann. 

 der Physii<, 61, p. 224, 1897. 



