46 Prof. J. Clerk Maxwell on [Dec. 18, 



value of TT to 500 decimals and the date of its publication, was correct, 

 as such value was compared with the author's given in Mr. Rutherford's 

 paper of 1853, and of course agrees with what is given above in this. 



IV. <f On Double Refraction in a Viscous Fluid in motion." By 

 J. CLERK MAXWELL, M.A., Professor of Experimental Physics 

 in the University of Cambridge. Received October 31 , 1873. 



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 shown 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 ob- 

 served 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 of polarization, is inclined 45 to it, no effect is observed, showing 

 that the axes of strain are inclined 45 to the plane of shearing, as indi- 

 cated by the theory. 



I am not aware that this method of rendering visible the state of strain 



* Journal de 1'ficole Polytechnique, tomexiii. cah. xx. (1829). 



