394 PROCEEDINGS OF THE AMERICAN ACADEMY. 





A,„ A — A,„,' 



If m and rrii were each assigned the same value obtained by dividing 

 our original value by 2, in all probability a very close approximation 

 would be obtained in the region in question. This has not been done for 

 two reasons. In the first place it does not appear as if much would be 

 learned by the procedure, and in the second place m and m' are not 

 equal, as is shown by the stronger dispersion near D.,, and until the rela- 

 tive-values have been determined we are not in a position to write the 

 two-member formula accurately. It is doubtful whether anything new 

 would come out of such a determination, and it was on that account not 

 attempted. 



Another matter of considerable interest is the question of the indices 

 represented by the square root of a negative quantity in the immediate 

 vicinity of the absorption band on the blue side. Lord Kelvin inter- 

 prets this as indicating that no light of such wave-lengths enters the 

 medium ; in other words, it is metallically reflected. 



It is in this way that he lias explained the apparent greater broaden- 

 ing of the D lines on the more refrangible side in some of Becquerel's 

 photographs. In the case with which we are dealing the second term of 

 our original formula does not become less than unity until we reach 



?n A^ 



wave-length 58898, which we get by equating -^ — ^ to unity and 



A — A„,- 



solving for A. 



This shows us that, even with a vapor so dense that both D lines run 

 together and broaden out into a wide band, we do not get values of the 

 index which are imaginary until we are within 0.2 of an Angstrom unit 

 of the D line, or in other words until we are within a distance of I) equal 

 to ^jj of the distance between Di and D2. 



In the case of the comparatively rare vapor employed by Becquerel 

 we should have to approach much closer than this to get the imaginary 

 values. This makes it appear certain that the greater broadening on tlie 

 more refrangible side, if it exists, must be assigned to some other cause 

 than imaginary values of the refractive index. 



The medium is exceptionally interesting in that its dispersion can be 

 represented throughout the entire range of wave-lengths without taking 

 the extinction coetFicient into account, as is always necessary in the case 

 of solids and li(j[uids when in the vicinity of the absorption band. 



