748 



Prof. E. G. Coker on the 



As an example of a more complicated kind we may take a 

 hook of very great curvature. This case differs optically 

 from the preceding one in a very important way, as the 

 principal stresses on each side of the principal section show 

 considerable variation in angular position. 



If, therefore, a plane-polarized beam passes perpendicularly 

 through the plate with the intersection of its plane of polari- 

 zation oblique to the directions of principal stress, it is 

 resolved into components corresponding to these latter 

 directions, and therefore in all parts of the specimen through 

 which the plane-polarized light passes, the ray is resolved 

 into two directions at right angles. The interference of 

 these two rays produces colour fringes, except in those parts 

 of the field where the directions coincide with the planes 

 of the crossed nicols. Such bands, therefore, indicate the 

 directions of the principal stresses, and by turning the nicols 

 round while their planes of polarization remain at right 

 angles to one another, a series of curves are obtained as 

 shown, for example, by fig. 4, where these loci have been 



obtained for a hook having an outer radius of 0*75 in. and 

 an inner radius of 0'277 in., the plate being 0*123 in. 

 thick. 



The effect thus produced is equivalent to superposing a 

 black cross upon the interference colours produced by the 

 stress, and it serves a useful purpose in that it enables curves 

 of principal stress to be drawn graphically, or by calculation 



