456 Mr. S. S. Richardson on Magnetic Rotary 



Thus 



where the terms under the summation sign refer to ultra- 

 violet resonators only (electrons). Drude (loc. cit.) shows 

 that if we confine our attention to this case, we may 

 determine a lower limit to the number of electrons affecting 

 the refractive index. If # l9 x 2 denote the reciprocals of the 

 squares of two wave-lengths in the visible or ultraviolet 

 ;and ?\, r 2 the values of 



n 2 -l + c\ 2 



for these wave-lengths, and if 



dt 



x 2 



then the lower limit referred to is given by the inequality 



;M TlT 2 



m 



(Pl+i>2 + 



.j^9-78xl0< 



du 



(3) 



Drude has applied equation (3) to the data for a large 

 number of organic and several inorganic compounds, and 

 arrives at the conclusion that "the number of electrons in 

 the molecule which influence the refractive index is equal 

 to the sum of the valencies of the several atoms in the 

 molecule." 



Similar calculations have been made by Erfle for a number 

 of aromatic and other unsaturated compounds. 



The following table will suffice to indicate the general 

 character of the numerical results. 



Table I. 



(Drude.) 



v. 



p. 



(Erfle.) 



v. 



p. 



Fluorite 



4(16) 

 8(16) 

 8(16) 

 4 (8) 

 2 (8) 

 2 (8) 



10(14) 



16 (20) 



30 



36 



42 



92 (136) 



9-5 



8-2 



73 



3-9 



7'6 



5-6 



8-4 



12-5 



12-0 



14-8 



176 



149 



Amylene 



Propargyl alcohol 



30 



18 (22) 

 12 (30) 

 48 

 42 (44) 



8(32) 



136 (164) 



40 (56) 



8(14) 



4 



2 



6(10) 



15-3 



9-9 



15-7 



13-9 



23-3 



21-6 



905 



29-9 



5-5 



2-7 



1-9 



4-5 





Carbon bisulphide ... 

 Water 



Tribrom-etbylene 



Naphthalene 



Sylvine 



Triethylarnine 





Silicon bromide 



Methyl alcohol 



Hexapropyl disilicate ... 



Tetramethyl silicate 



Hydroxylamine 



Ethyl alcohol 









Xylene 



Hydrogen 





Nitrogen 







