Scattering and Dispersion of Light. 829 



The reflexion from the face of a deep imperfect crystal is 

 evaluated (§ 7), and it is shown how the secondary extinc- 

 tion may be eliminated. 



The same process for reflexion from the interior planes of 

 a plate is worked out (§ 8), and the formulae are justified 

 whereby Bragg, James, and Bosanquet eliminated the 

 extinction — but only the secondary extinction. 



The theoretical results are compared with experiment (§ 9). 

 The experimental data are rather inadequate and the agree- 

 ment is not very good. 



The corresponding calculations are done for the powder 

 method of observation on crystals (§ 10). 



The paper concludes with a short discussion (§ 11), 

 suggesting the need of further tests. 



XOIII. Scattering and Dispersion of Light. By U. Dor, 

 Research Student in the Lnstitute of Physical and Chemical 

 Research, Tokyo *. 



AUTHORS differ in their opinions as to the mechanism 

 of scattering light by a medium through which the 

 light travels. Schuster asserts, however, in his ( Theory 

 of Light ' (p. 325) that, if a molecule of the medium may be 

 looked upon as a separate source of scattering, the scattering- 

 due to it follows undeviatedly the celebrated formula 

 of Lord Rayleigh, whatever be the theory we adopt. It 

 will not be without interest, for instance, to notice that 

 Jakob Kunzf, indeed, derived exactly the same formula 

 from an elementary theory of scattering of light by small 

 dielectric spheres. 



Ever since the electron theory of matter began its 

 striding progress, and the well-known dispersion formula 

 was deduced by H. A. Lorentz through his electronian 

 analysis of atomic constructions, attempts have been made 

 to interpret the absorption of light from the electronian 

 standpoint of view. Thus, Drude J and Voigt § attribute 

 it to the damping of the oscillations of bound electrons 

 in the atoms of the absorbing medium, the damping pro- 

 cess being caused by a resisting force proportions 1 to 

 the velocity of the electrons. They insert consequents 

 a term of this damping in the equation of motion of 



* Communicated bj^ the Author. 



t Phil. Mag. xxxix. p. 416 (1920). 



£ P. Drude, Lehrbuch cler Optik, p. 353. 



§ W. Voigt, Magneto- unci Electrooptik, p. 104. 



