480 8. P. Langley — Optical Properties of Rock-salt. 



venience of others having occasion to work with this material, 

 and for testing theories of dispersion. 



Eefracting Angle of Prism = 59° 57' 54". 



jine. 



Wave-length. 



Deviation. 



A 



Refractive Index, 



M 



0-3727 



43° 50' 57" 



1-21 



1-57486 



L 



0-3820 



43 35 27 



1.20 



1-57207 



H 2 



0-3933 



43 19 32 



1-19 



1-56920 



H, 



0-3968 



43 14 44 



1-19 



1-56833 



G 



0-4303 



42 36 7 



1-16 



1-56133 



F 



0-4861 



41 51 47 



1-13 



1-55323 



\ 



0-5167 



41 33 43 



1-12 



1-54991 



\ 



0-5183 



41 32 52 



1-12 



1-54975 



D, 



0-5789 



41 2 41 



1-10 



1-54418 



r> 2 



0-5895 



41 2 29 



1-10 



1-54414 



c 



0-6562 



40 42 56 



1-09 



1-54051 



B 



0-6867 



40 35 49 



1-09 



1-53919 



A 



0-760,1 



40 22 25 



1-08 



1-53670 



p6t 



0-94 



40 1 26 



1-07 



1-5328 



9 



1-13 



39 49 11 



1-06 



1-5305 



tp 



1.39 



39 39 56 



ro5 



1-5287 



n 



1-32 



39 29 21 



1-05 



1-5268 



Temperature = 24° C. Barometer 731-l mm . 



The wave-lengths of the M and L lines are from Cornu, 

 those of the lines between H and A inclusive, from Angstrom, 

 and those of the infra-red bands from the Allegheny observa- 

 tions. The column headed J was prepared at the suggestion 

 of Mr. Keeler, and is for the purpose of facilitating the re- 

 duction of observations made with a different prism angle from 

 that for which the table is computed, and for which our wave- 

 length curves are drawn. If we differentiate the ordinary 

 formula for a prism 



_sin %(A + d) 



sin 



A 



with respect to A, which we now regard as a variable, we have 



dd _ n cos -JA _ 



dA~cos i(A + d) ~ ~ 

 or dd=Jd A. 



The values of A for the different lines of the spectrum are 

 readily computed from the table of deviations and refractive 

 indices. To find, then,, the deviation of a line after any re- 

 polishing of the prism, we have merely to multiply the change 

 of the angle by the approximate value of J taken from the 

 table, and we obtain the change in the deviation of the line, 

 and hence also the deviation required. Thus, if the new angle 

 is found on measurement to be 59° 57' 44", c?A= — 10", and 



