IN PLATES, TUBES, AND CYLINDERS OF GLASS. 365 



5. On the Conversion of Plates of Glass with One Axis into 

 Plates with Two Axes. 



From the different experiments which I have described, 

 both in this and in other papers, it appears, that in almost 

 every case where polarising forces are developed, two negative 

 structures are separated by a positive structure, or two positive 

 structures by a negative structure, both of which are simulta- 

 neously produced, like the two opposite polarities in electrici- 

 ty and magnetism. When the two structures are disposed in 

 a different manner, or when only one structure is developed, 

 the regular arrangement of the polarising forces may be effec- 

 ted by a slight change in the form or in the mechanical condi- 

 tion of the plate. 



If we take a piece of unannealed glass perfectly circular, we 

 have only one axis, or one structure, as shewn in Fig. 10., where 

 AB, CD is the black cross, which preserves the same appearance 

 by turning the plate round its centre. But if we grind the 

 smallest quantity from any two opposite sides CB, AD, so as to 

 induce a slight degree of ellipticity, a new axis is created ; the 

 tint which it produces appears at the centre of the plate, and the 

 system of rings has the form shewn in Fig. 8., where the inter- 

 nal structure within the black fringes AD, CB, is negative, and 

 the two external structures without AD, CB, positive. 



If, on the contrary, we now grind a small quantity from 

 the sides AC, BD, so as to reconvert the elliptical plate into a 

 circular plate, we shall find that the new axis which was gene- 

 rated by the change of form, has entirely disappeared, so that 

 the plate exhibits the figure shewn in Fig. 10. 



This singular experiment, in which one of the axes may be 

 extinguished and revived at pleasure, is worthy of the most 



attentive 



