[From the Proceedings of the Royal Society, No. 148, 1873.] 



LXIII. On Double Refraction in a Viscous Fluid in Motion. 



ACCORDING to Poisson's* theory of the internal friction of fluids, a viscous 

 fluid behaves as an elastic solid would do if it were periodically liquefied for an 

 instant and solidified again, so that at each fresh start it becomes for the 

 moment like an elastic solid free from strain. The state of strain of certain 

 transparent bodies may be investigated by means of their action on polarized 

 light. This action was observed by Brewster, and was shewn by Fresnel to be 

 an instance of double refraction. 



In 1866 I made some attempts to ascertain whether the state of strain in 

 a viscous fluid in motion could be detected by its action on polarized light. I 

 had a cylindrical box with a glass bottom. Within this box a solid cylinder 

 could be made to rotate. The fluid to be examined was placed in the annular 

 space between this cylinder and the sides of the box. Polarized light was 

 thrown up through the fluid parallel to the axis, and the inner cylinder was 

 then made to rotate. I was unable to obtain any result with solution of gum or 

 sirup of sugar, though I observed an effect on polarized light when I compressed 

 some Canada balsam which had become very thick and almost solid in a bottle. 



It is easy, however, to observe the effect in Canada balsam, which is so 

 fluid that it very rapidly assumes a level surface after being disturbed. Put 

 some Canada balsam in a wide-mouthed square bottle ; let light, polarized in a 

 vertical plane, be transmitted through the fluid ; observe the light through a 

 Nicol's prism, and turn the prism so as to cut off the light; insert a spatula 

 in the Canada balsam, in a vertical plane passing through the eye. Whenever 

 the spatula is moved up or down in the fluid, the light reappears on both 

 sides of the spatula ; this continues only so long as the spatula is in motion. 

 As soon as the motion stops, the light disappears, and that so quickly that I 

 have hitherto been unable to determine the rate of relaxation of that state of 

 strain which the light indicates. 



If the motion of the spatula in its own plane, instead of being in the plane 



* Journal de VEcole Polytechnique, tome xiii. cah. xx. (1829). 



482 



