Blrefringent Action of Strained Glass. 



341 



(2) Compression along A B. As the strain increases from 

 zero, the wave-surface for the given time contracts, and breaks 



up into two sheets, represented by the two inner curves, 

 ellipse and circle, the ellipse touching the new circle in points 

 of A B, and the primitive circle in points of C D. 



(3) Tension along A B. As the strain increases from zero, 

 the wave-surface for the given time expands, and breaks up 

 into two sheets, represented by the two outer curves, ellipse 

 and circle, the ellipse touching the new circle in points of A B, 

 and the primitive circle in points of C D. 



19. The matter of last article may be presented otherwise 

 thus, in the ordinary language of Physical Optics. Glass in 

 a state of directional strain acts on light as a uniaxal crystal, 

 with optic axis parallel to the line of strain, the ordinary index 

 and the extraordinary being equal to ?n(l + a) and m respec- 

 tively, where the small number a is positive in the case of 

 compression, negative in the case of tension, and (it should be 

 added ) sensibly proportional to the strain. 



The elasticities of the medium, (1) in the absence of strain, 

 (2) along the line of strain, (3) at right angles to that line, 

 are as the numbers 1, 1, 1 — 2a, where the small number a 

 (as above) is positive in the case of compression, negative in 

 the case of tension, and sensibly proportional to the strain. 

 It is surely not an accident, that these optical properties of 



