Dispersion of Metals. 663 



surface of different samples of the same material. There 

 seems to be, at least tacitly, a general opinion that since 

 D rade's specification of the u normal " condition for a 

 reflecting surface, it only requires sufficient care in the 

 preparation of a metallic mirror in order to get consistent 

 results ; and that, consequently, our knowledge of the optical 

 constants is nearly as precise as that of the methods of 

 measurement used. It is safe to say, however, that no one 

 who has had much experience in this line of work has held, 

 such an opinion for long. In fact, experience teaches that 

 it is well nigh impossible to exactly reproduce (even by the 

 same observer) the same condition of the reflecting surface 

 in different samples of the identical material, or even in the 

 same sample at different times. Thus we cannot regard the 

 experimental values of the optical constants of metals as 

 anything but rather rough approximations to the specific 

 values characteristic of the substances. Hence any theo- 

 retical relation dependent for its numerical evaluation on 

 the absolute values of the optical constants is incapable of 

 yielding dependable results. 



On the other hand, since, as has been pointed out above, 

 the data at our command as to the relative values of the 

 optical constants at different wave-lengths are in general of 

 a higher order of precision, it would seem that a comparison 

 of these results with the dispersion formulae yielded by the 

 electron theory should yield results of some value. To make 

 such a comparison is the object of this paper, 



It may be mentioned in passing,, thar it would seem to be 

 possible to obtain accurate absolute values really repre- 

 senting specific properties of the substance in the case of 

 mercury ; for the author has shown that in the case of this 

 metal it is possible to reproduce exactly * surface conditions 

 entirely free from all films of impurity. The possibility of 

 different states of surface strain at different times would also 

 of course be eliminated. Work on the dispersion of mercury 

 is now under way in order to obtain with this more promising 

 material, data which will enable us to make a more satis- 

 factory comparison with theory than is possible with other 

 substances. 



With regard to the properties of the metals revealed by 

 the data under discussion, it will be observed that all live of 

 the substances show in general a decrease of the absorption 

 coefficient (increase in transparency) with increasing fre- 

 quency of the incident light. In the cases of silver and gold 



* "Wheeler, Phil. Mag. xxi. p. 229; also Am. Jour. Sci. xxxii. p. 65 

 (1911). 



2 Z 2 



