﻿On the Scattering of Light by small Particles. 4>4<7 



Water at 18°-9 C. gave u = 11° 35', d =09996 

 69°-0 „ m 1= =10° 35', ^=0-9799. 



when the tangent-balance was adjusted to the density of abso- 

 lute alcohol 07946. 



From this are obtained the volumes 



v =1-000400 

 * = 1-002051 



Av =0-001651 

 for A* = 50°-l C. 



The correction for the expansion of the glass rod is ^=0*00451, 

 and hence the actual expansion 



Av= 0-002102. 



According to Despretz, the volume of water at 



18°-9C. v =1-00156 

 69°-0 ^ = 1-02200 



dv =0-02044 

 Prague, December 17, 1870. 



LVIII. On the Scattering of Light by small Particles. By the 

 Hon. J. W. Strtjtt, Fellow of Trinity College, Cambridge*. 



THE investigation of the diffraction of light by small par- 

 ticles, contained in the February Number of this Maga- 

 zine, proceeds throughout on the assumption that the difference 

 between two media which differ in refractive power is a difference 

 of density and not a difference of rigidity. My object in the 

 present communication is to attack the problem more generally, 

 and to show that the more special hypothesis is in no degree 

 arbitrary, but forced upon us by the phenomena themselves. 

 The words " density," " rigidity " need not be interpreted lite- 

 rally, but are used in a generalized sense analogous to that given 

 to " velocity" and "force" in the higher mechanics. 



The first step is to find the equation of motion of an isotropic 

 elastic medium whose density and rigidity may vary from point 

 to point. If D denote the density and n the rigidity, a process 

 similar to that used in Thomson and Tait's { Natural Philosophy/ 

 p. 530, leads us to the following : — 



£M) + V„?-Dp 



dn drj dn d£ dn drj dn d% __~ .. . 



dx dy dec dz dy dx dz dx~~ ' ' 



* Communicated by the Author. 



