of Infra- Red Rays in Rock Salt and iSylvine. 41 



«t D . The second column gives the corresponding indices of 

 refraction. In the third are the observed wave lengths of the 

 rays without the application of the correction mentioned above, 

 and in the fourth, the same, corrected. In the fifth column 

 are the values of the wave lengths for the corresponding 

 indices of refraction, calculated from the Ketteler-Helmholtz 

 formula, using the constants given by Mr. Nichols and one 

 of US- 





Table I. 









Rock salt <j> 



— 10° 53' 



00" 





a — a„ 



D A 



n X 



uncorrected. 



A 



corrected. 



A 



calculated. 







1-5441* 



0-589/x 







0-589 







1-5030* 



8-67 







8-67 



32' 19* 



1-4951 



9-88 



9-95/x 



10 no 



41 55 



1-4805 



11-85 



11-88 



12 00 



53 39 



1-4627 



13-94 



13-96 



14 00 



i° r 55" 



1-4410 



15-81 



15-89 



16-00 



1 25 9 



1-4148 



17-87 



17-93 



18 00 





1-3735** 



20-57 







20-57 





1 3403** 



22-3 







22-3 



b 2 = 5-1790, 



M^ = 0-018 496 



3149-3. 



3977-0, X; 



= 0-01621, 





Table 11. 









Sylvine <p = 



= 12° 39' 10". 





a — a-. 



D A 



n 



uncorrected 



A 



corrected. 



A 



calculated 







14900* 



0'589/a 





0-5s7/x 







1-4653* 



7-08 







7-08 



26' 2" 



1-4561 



9-94 



10-01//, 



10-00 



41 40 



1-4362 



13-88 



14-14 



14 20 



1° 1 7 



1-4102 



17-94 



18*10 



18 05 







1-3882** 



2060 





20 60 







1-3692** 



22-5 





22 5 



b* = 4-5531 



, M 1 — O'OloO 



A x 2 = 0-0234, 3f m 



= 10747, 





v= 



4517-1. 







The calculated values of the wave lengths are, for both sub- 

 stances, almost without exception, larger than the observed 

 values ; however, the difference is, for the most part, less than 

 ■J- per cent, and this ^ percent lies generally within the limit of 

 error in observation, which must be estimated at from 0'3 to 

 0-7 per cent. Therefore the equation for the dispersion satis- 

 fies the demands made on it in the portion of the spectrum 

 between \ = 10/jl and A, = 20//,. 



*H. Rubens. Berichte der Physikal. Gesellschaft, 6 Nov., 1896. 



