POLARISATION OF LIGHT. 



55 



When the polarised light is transmitted 

 in a direction perpendicular to the axis, or 

 in the plane of the equator, the black cross 

 will be complete, and the figure sym- 

 metrical when the plane of the equator 

 is parallel or perpendicular to the plane 

 of primitive polarisation ; but out of these 

 planes the black cross changes its form, 

 and only two luminous spaces appears 

 in place V>f 1 2, 78, separated by a single 

 black line passing through the centre. 

 The middle and outer sectors of the 

 crystalline had the same structure as 

 that of a sphere of glass placed in hot oil, 

 and the middle sectors had an opposite 

 structure*. 



CHAPTER XV. 



Double Refraction communicated to 

 Plates of Glass by Mechanical Force 

 Combination of such Plates with 

 each other and with Plates of Glass 

 made Doubly Refracti?ig by Heat 

 Chromatic Dynamometer Manner 

 in which Heat and Pressure produce 

 Double Refraction. 



ON the 3d of January, 1815, Dr. Brew- 

 ster discovered that the property of 

 double refraction could be communi- 

 cated by simple pressure to soft animal 

 substances, such as isinglass and calves' 



foot jelly. He took a cylinder of calves' 

 foot jelly, so small that *it could scarcely 

 support its own weight, and having no 

 action whatever upon polarised light ; 

 and by pressing this between the finger 

 and the thumb, or even touching it 

 gently, it received the structure of doubly 

 refracting bodies as exhibited in their 

 action on polarised light. During sub- 

 sequent experiments in October, 1815, 

 he found that compression produced a 

 negative polarising structure, and dila- 

 tation a positive structure, and by di- 

 lating isinglass in a certain state of 

 toughness, he communicated to isinglass 

 a much more powerful doubly refracting 

 structure than that of beryl. On the 

 1st of November of the same year, he 

 extended these experiments to plates of 

 solid glass. 



This remarkable property may be 

 easily shown by taking a strip of glass 

 cut merely with a diamond, and bending 

 it slightly by holding one end of it in 

 each hand. When the strip of glass is 

 held in the apparatus,^. 39, so that its 

 length is inclined 45 to the plane of 

 primitive polarisation, it will exhibit 

 two separate doubly refracting structures 

 shown in fig. 65, separated by the dark 

 neutral line M X. Each of these struc- 



Fig. 65. 



tures is covered with coloured fringes 

 parallel to M N, and those between the 

 concave side C D and M N are positive, 

 while those between the convex side, 

 A B and M X, are negative. The tints 

 vary as their distance from M X. If we 

 slacken the bending force, the fringes 

 will become less numerous, descending to 

 the white of the first order, and then 

 disappearing altogether when the force 

 is reduced to nothing. 



When two slips of glass of the same 

 thickness and si^e, and similarly bent, 

 are crossed, as shown in Jig. 66, the 

 tints in the intersectional square are 

 rectilineal, and are parallel to the line 

 of no double refraction m n, which forms 

 the diagonal of the square, which joins 

 the intersection of the two concave sides 

 with that of the two convex sides. 



When one of these bent strips of glass 



* See Phil. Trans. 1816,p.311, where Dr. Brewster 

 first described these appearances. 



crosses a rectangular plate of glass with 

 the two structures, the fringes in the 

 intersectional square are parabolas. 



Fig. 66. 



Effects of a similar kind were pro- 

 duced by applying mechanical force to 

 various other bodies destitute of double 

 refraction ; and the system of rings in se- 

 parate crystals, and the uniform tints pro- 



