362 ON THE DISTRIBUTION OF THE POLARTSING FORCE 



When T — T' the line of no-polarisation is a complete para- 

 bola, passing through the angles of the parallelogram, and ha- 

 ving its vertex at the very edge of the bent plate. The curves 

 within it will also be complete parabolas, but all those without 

 it will be only parabolic segments. 



4. On the Distribution of the Polarising Force in Tubes and 



Cylinders of Glass. 



We have already seen, that circular plates, or cylinders of 

 glass, have one negative axis, like Quartz ; but when the cylin- 

 der has the form of a tube, like AB, Plate VII. Fig. 3. the pola- 

 rising force is distributed in a very remarkable manner. A 

 black circular fringe mpno forms the line of no-polarisation, 

 and the coloured fringes are placed on each side of this dark 

 ring, and concentric with it. The structure on the outside of 

 mp n o is negative like Calcareous spar ; and the structure on the 

 inside positive, like Quartz, &c. ; and the effect is exactly the 

 same as if a plate of glass had been bent into a circular form, 

 and kept in that position by force. 



The breadth of the positive annulus a o is always less than 

 that of the negative annulus A o, the former decreasing, and 

 the latter increasing, as the bore of the tube diminishes ; and 

 when the bore becomes extremely small, as in Fig. 4. the posi- 

 tive structure is also extremely small, and sometimes can 

 scarcely be seen without the aid of a microscope. 



In comparing the values of the tints with their distances 

 from the line mp n o, I have found that they vary as the distan- 

 ces, in the same manner as in bent glass. Hence it is obvious, 

 that the glass is in a state of compression within the black 

 rino- mpn o, and in a state of dilatation without that ring, and 

 that the particles are held in a state of violent constraint, en- 

 tirely different from that position of equilibrium in which they 

 are placed in plates of glass with one or two axes. 



With 



