Dispersion of Diamond. 405 



Now, substituting in (11) the value (9 a) of a, and putting 



M = 12, and the density of diamond, cZ = 3*515 (the mean of 



the best measurements *), we find as the atomic coefficient of 



k 

 carbon, entering into its "atomic re£ractivity " C= — ° , 



Uq — u 



& =4'07 . 10 10 cm. gr." 1 . . . . (12a) 



This exceeds the nearest multiple (2e) of the proper electronic 

 value by as much as 0*22 e, a feature certainly not agreeable 

 to the pan-electronists. I do not propose here to attempt 

 to bring it down to an exact multiple of e by some artificial 

 -assumptions. 



The other atomic attribute of carbon, its free wave-length* 

 would be, by (9 a), 



X =1142A.U., (126) 



-as already quoted. There is nothing unlikely about this 

 latter wave-length. Diamond has been known since 1862 

 (Miller, Phil. Trans, clii. pp. 861-887, quoted by Martens) 

 to be o transparent for ultraviolet rays as far down as 

 2240 A.U. I do not know whether this crystal has been 

 investigated for its absorption below this limit. It would 

 be very interesting to undertake experiments in the region 

 2240-^1000 A.U. with the considerably improved modern 

 means. 



It will be kept in mind that the free wave-length (12 b) 

 belongs to carbon inasmuch as we accept the absence of 

 resultant interaction between the carbon atoms, which after 

 what was said above it seems to me we have all reasons to 

 accept. On the other hand, if (as we may abstractedly 

 assume, for the sake of shedding some light upon the 

 problem) there were some interaction, we should ultimately 

 obtain a formula which would again be of the type of (9), 

 viz. 



,J-1=-?—, (13) 



r Uq — U 



the only difference consisting inj modified value of the 

 "free wave-length" (\ ' = 1 : \A</)- Tnis statement can 

 easily be proved. 



In fact if there is some resultant interaction, i. e. if 

 12t£0, we have, by (1), with 12 as explained in (2), with 

 «equal p; = p, 



* Martens does not quote the density of his piece of diamond. 



