Refraction in a Viscous Fluid in Motion. 891 



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 ob- 

 served, showing that the axes of strain are inclined 45° to the plane 

 of shearing, as indicated by the theory. 



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

 strain of a viscous fluid has been hitherto employed ; but it appears 

 capable of furnishing important information as to the nature of vis- 

 cosity in different substances. 



Among transparent solids there is considerable diversity in their 

 action on polarized light. If a small portion is cut from a piece of 

 unannealed glass at a place where the strain is uniform, the effect 

 on polarized light vanishes as soon as the glass is relieved from the 

 stress caused by the unequal contraction of the parts surrounding it. 



But if a plate of gelatine is allowed to dry under longitudinal 

 tension, a small piece cut out of it exhibits the same effect on 

 light as it did before, showing that a state of strain can exist 

 without the action of stress. A film of gutta percha which has 

 been stretched in one direction has a similar action on light. If 

 a circular piece is cut out of such a stretched film and warmed, it 

 contracts in the direction in which the stretching took place. 



The body of a sea-nettle has all the appearance of a transparent 

 jelly ; and at one time I thought that the spontaneous contractions 

 of the living animal might be rendered visible by means of polarized 

 light transmitted through its body. But I found that even a very 

 considerable pressure applied to the sides of the sea-nettle pro- 

 duced no effect on polarized light, and I thus found, what I might 

 have learned by dissection, that the sea-nettle is not a true jelly, 

 but consists of cells filled with fluid. 



On the other hand, the crystalline lens of the eye, as Brewster 

 observed, has a strong action on polarized light when strained 

 either by external pressure or by the unequal contraction of its 

 parts as it becomes dry. 



I have enumerated these instances of the application of polarized 

 light to the study of the structure of solid bodies as suggestions 

 with respect to the application of the same method to liquids so 

 as to determine whether a given liquid differs from a solid in 

 having a very small " rigidity," or in having a small " time of re- 

 laxation"*, or in both ways. Those which, like Canada balsam, 

 act strongly on polarized light, have probably a small " rigidity," 

 but a sensible " time of relaxation." Those which do not show this 

 •action are probably much more " rigid," and owe their fluidity to 

 the smallness of their " time of relaxation." 



* The " time of relaxation " of a substance strained in a given manner is the 

 time required for the complete relaxation of the strain, supposing the rate of 

 relaxation to remain the same as at the beginning of this time. 



