364 ON THE DISTRIBUTION OF THE POLARISING FORCE 



ternal tints do not rise so high, from the diminution in the 

 thickness of the glass. 



The same phenomena are exhibited by a glass-tube AB, 

 Fig. 6. similarly placed. In this case, however, the maximum 

 tint does not appear along the line m ra, the axis of the tube ; 

 but it is seen both in the lines b b and d d, equidistant from 

 m m. This effect is obviously occasioned by the greater thick- 

 ness of the glass in these directions ; for a b and c d are each 

 greater than Ao + » m, and the difference is sufficiently great 

 to overbalance the diminution of the tints at a greater distance 

 from the principal axis. 



In examining very carefully the structure of glass tubes, 

 when exposed to polarised light, it will be found that they are 

 as it were divided into different elementary concentric tubes ; 

 and that in some cases there is an actual separation between 

 them. Hence, there arises a remarkable singularity in the 

 progression of the tints. Instead of shading into one another 

 by imperceptible gradations, each elementary tube has an uni- 

 form tint of its own, as is represented in Fig. 7. where the 

 tube AB is divided into four tubes 1,1; 2,2 ; 3, 3 ; 4, 4 ; the 

 tube 4, 4 is in every part of it a white of the first order ; the 

 tube 3,3 is every where equally dark, being the black cir- 

 cular fringe mpno of Fig. 3. ; the tubes 2, 2 and 4, 4 are 

 a white of the first order ; and the tube 1,1 is a yellow of the 

 first order. Hence it follows, that in tubes which possess this 

 peculiarity of structure, each elementary tube is uniformly 

 dense throughout, and that the variation of density takes place 

 by leaps. 



If a portion of a glass tube perfectly annealed, is exposed to 

 pressure, it exhibits the tints shown in Fig. 8. when the line 

 m 7i, joining the points of pressure, is parallel or perpendicular 

 to the plane of primitive polarisation ; and the tints shown in 

 Fig. 9. when the same line is inclined 45° to that plane. 



5, On 



