380 DOUBLE REFRACTION IN A VISCOUS FLUID IN MOTION. 



of polarization, is inclined 45* to it, no effect is observed, shewing 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 viscosity 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, 

 shewing 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 produced 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 relaxation"*, 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 shew 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 tliii time. 



