330 



MESSRS. C. CUTHBERTSON AND E. B. R. PRIDEAUX 



This prediction has been verified. The refractivity of chlorine for sodium light is 

 generally accepted to be 768, or 4x192. That which is now found for fluorine 

 is 195. 



Table IV. shows the exact ratios experimentally obtained in all cases. The indices 

 were determined, in the case of the inert gases for white light, in that of iodine for 

 the red and the violet. In all other cases the measurements are for sodium light. 



It will be seen that, except in the case of the anomalous red rays in iodine, the 

 discrepancies between the ratios actually found and those of integers do not exceed 

 3 '2 per cent. A discussion of the possible causes of these discrepancies will be found 

 in Mr. CUTHBERT.SOX'S paper. 



The element chosen as standard in each group is indicated by an asterisk, and, to 

 avoid doubling all the other figures, the ratio of helium is taken as one half. 



TABLE IV. 



With the addition of fluorine the table given in the paper quoted above, showing 

 the refract ivities of all the elements whose index has been measured in the gaseous 

 state, now stands as follows. A few additional elements are put down to suggest the 

 framework of the periodic system, and the refract ivities are rounded off. 



