Birefringent Action of Strained Glass. 327 



plates (1) when the glass is compressed and the crystal 

 negative, and (2) when the glass is extended and the crystal 

 positive. 



Suppose optic axis and line of strain both vertical. Let 

 and E be the retardations of the ordinary and extraordinary 

 pencils in the crystal, V and B7 the strain-generated retarda- 

 tions of the same pencils (V and H)*in the glass. We know 

 that the sign of the difference E — is the nominal sign of 

 the crystal. The relative retardation H' — V is therefore nega- 

 tive in the case of vertical compression, and positive in the case 

 of vertical tension. But it follows from the two preceding 

 propositions, that strained glass must act in the polariscope 

 according to these very laws ; for (1) in the case of vertical 

 compression, V > H', 



... H'-V'<0; 

 and (2) in the case of vertical tension, V^H^O, 

 .-. H'-V'X). 



According to Brewster's usual manner of putting his facts, 

 the extraordinary ray is relatively accelerated in negative 

 crystals, and relatively retarded in positive crystals ; and 

 similarly, in glass vertically strained, the component pencil H 

 is relatively accelerated in the case of compression, relatively 

 retarded in the case of tension. What was thus presented to 

 Brewster, in the case of glass vertically compressed, as a rela- 

 tive acceleration of the component pencil H, is now exhibited 

 more definitely, as an absolute and excessive retardation of the 

 other component pencil V ; and similarly, in the case of 

 vertical tension, the relative retardation of H is now exhibited 

 as an absolute and excessive acceleration of V. 



These results are independent of all hypotheses ; they 

 neither presuppose nor exclude any particular laws of refrac- 

 tion, ordinary or extraordinary, in the strained medium. 

 The experimental interpretation of Brewster's property will 

 be simplified and completed further on. 



7. The winged plate. This new piece is shown in two 

 forms in the diagram on the following page. 



It is a parallel-surfaced plate of glass, i to ^ inch thick, 



divided into a central pillar and two wings, by slits ground 



through the plate for the greater part of its length, like fine 



-slits through a plank. The central pillar is, as nearly as 



Me, a rectangular parallelepiped, its two ends being ground 



* Not the V and II of art. 3, but similarly named for the same 

 purpose, that is, to indicate briefly and clearly the directions of their 

 planes of polarization. 



