POLARISED LIGHT 361 



effect of strains of another character, resembling that pro- 

 duced by pressure on the centre of a beam. 



Clear jelly compressed from above in a glass trough, will 

 produce similar effects ; but a simpler experiment is to cut a 

 strip of thin india-rubber about 2 \ inches wide, pass it from side 

 to side through the stage, roll each end round a rod of wood, 

 and then strain the ends apart. The best effect is of course 

 in the 45 position, as the polarising planes are in the direc- 

 tions of greatest and least strain. A simple bar of glass 

 about half an inch square, passed through the stage, can 

 easily be bent with the fingers alone, so as to show coloured 

 fringes upon the screen. 



Heating a piece of glass from one point, by the local 

 expansion and strain it produces, exhibits the same effects. 

 A disc of glass may be heated in a pair of forceps over a 

 spirit-flame (keeping the flame at the same point only) and 

 placed in the stage ; the dark field will be illuminated by 

 fringes. A better method is to provide a rectangular sheet 

 iron shell open att both ends, and with a square aperture 

 through each of its sides. In this can be placed a square oi 

 thick glass held in a bottom bar of wood, so that the plate 

 stands between the two apertures ; when a nearly red-hot bar 

 of iron can be pushed in to rest on the top edge of the square. 

 Beautiful fringes will appear instantly. 



In plates of glass highly heated and suddenly chilled 

 round the edges, these effects of strain are permanent and 

 very brilliant. Many shapes are procurable, but the simpler 

 ones of square, circle, oval, and triangle are most instructive ; 

 and the oval particularly so, as illustrating by analogy the 

 phenomena of a bi-axial crystal. 



Nearly all massive pieces of glass commonly procurable, 

 such as ink-stands, glass stoppers, paper-weights, &c., show 

 these latter phenomena. A section of thick glass tube, 

 ground and polished, rarely fails ; and many optical lenses show 

 a conspicuous black cross, so that the lenses of the polariscope 



