146 REPORT ON PHYSICAL OPTICS. 



a thin film between two plates of glass. This film had a single 

 axis of double refraction at every point in the direction of the axis 

 of pressure ; and the tint developed depended solely on the in- 

 clination of the ray to this line. Sir David Brewster has drawn 

 from this phenomenon some highly interesting conclusions respect- 

 ing the origin of double refraction in regular crystals. He men- 

 tions several facts which seem to prove that this property is not 

 inherent in the molecules themselves ; and he conceives that it is 

 developed by the unequal pressure caused by the forces of aggre- 

 gation, which are in general different in the direction of three 

 rectangular axes. Thus the double-refracting properties and the 

 crystalline form are referred to the same agency.* 



Sir David Brewster and Dr. Seebeck had before observed the 

 phenomena arising from unequal condensation and rare faction in 

 the case of uncrystallized bodies unequally heated. These pheno- 

 mena may be studied by applying a bar of hot iron to the edge 

 of a rectangular plate of glass, and placing it in the polarizing ap- 

 paratus, so that the heated edge may form an angle of 45 with 

 the plane of primitive polarization. At the end of some time the 

 whole surface of the plate is observed to be covered with coloured 

 bands, the parts near the opposite edges having acquired a positive 

 double-refracting structure, and those near the centre a negative 

 one. The effects are reversed when a plate of glass uniformly 

 heated is rapidly cooled at one of its edges ; and all the appear- 

 ances vanish when the glass acquires the same temperature 

 throughout.! These phenomena maybe endlessly varied by vary- 

 ing the form of the glass to which the heat is applied. If now, by 

 any means, the glass be arrested in one of these transient states, 

 it will have acquired a permanent double-refracting structure. 

 This has been accomplished by M. Seebeck by raising the glass to 

 a red heat, and then cooling it rapidly at the edges. As the outer 

 parts, which are thus more condensed, assume a fixed form in cool- 

 ing, the interior parts must accommodate themselves to that form, 

 and therefore retain a state of unequal density. The law of the 

 change of density, and therefore the double-refracting structure, 

 will depend on the external form ; and M. Seebeck found, accord- 

 ingly, that the coloured bands and patches which such bodies 

 display in polarized light, assume a regular arrangement which 



PA*ZVi..l880. t PAH. ZVM. 1816. 



