POPULAR LECTURES AND ADDRESSES. 



Now look at this table (Table III.) of refractive 

 indices, and you see that the difference of velocity 



TABLE III. TABLE OF REFRACTIVE INDICES. 



The numbers in the first two columns were determined by Dr. 

 Hopkinson, those in the last three by Messrs. Gladstone and Dale. 

 The index of refraction of air for light near the line E is I '0.10294. 



of red light A, and of violet light H, amounts 

 in carbon disulphide to i/i/th ; in dense flint 

 glass to nearly i/3Oth ; in hard crown glass to 

 I /73rd ; and in water and alcohol to rather more 

 than i/iooth. Hence, none of these substances 

 can have so many as 20 molecules in the 

 wave-length, if dispersion is to be accounted for 



