CHAP. V., 5.] 



OPTICS. SIR DAVID BREWSTER. 



115 



solids not usually possessed of any polarizing action) 

 acquired such properties by being suddenly and un- 

 equally cooled; and on the other, that such sub- 

 stances undergoing partial changes of temperature, 

 possess the same property. These phenomena are 

 exceedingly beautiful, and easily displayed. It is 

 sufficient to place a rectangular piece of glass, some- 

 what thick, with one of its longer edges in contact 

 with a hot iron, and placing it between a polarizing 

 and analyzing plate, to incline the heated edge 45 

 to the plane of polarization. Bands of light and 

 shade are seen to traverse the glass parallel to the 

 same edge, and simultaneously appear also on the 

 side farthest from the heated metal. They pass gra- 

 dually into rich coloured tints diffused with geometric 

 regularity. In the case of cubes or cylinders of glass, 

 suddenly, and therefore unequally cooled, the pheno- 

 mena are permanent, and the colours splendid, being 

 arranged in patterns which may be made to resemble 

 those of natural crystals. 



(528.) Sir David Brewster at first ascribed these effects 

 nd . in to the direct effect of the heat in the glass upon 

 cted to " ^&ht, an< ^ compared its simultaneous influence over 

 >ressure. a whole plate to a polar influence. Another curious 

 discovery, also due to him, leads to the simpler con- 

 clusion, that in every case the development of a 

 polarizing structure is due to the varying tension 

 (transient or permanent) into which the particles of 

 the glass are thrown by local expansion or by irre- 

 gular cooling. This discovery was, that similar 

 effects may be produced in jellies and soft transparent 

 substances by the effect of pressure, and that even 

 glass itself, when strained in any way by mechanical 

 force, shows depolarizing bands ; and crystals may 

 have their peculiar optical phenomena altered by 

 pressure. Fresnel, not satisfied with inferring that 

 the chromatic display is due to a doubly-refracting 

 structure communicated to the glass, contrived, by 

 an ingenious arrangement of prisms, actually to 

 exhibit the separation of the images under the action 

 of powerful pressure. 



(529.) III. Possibly the most remarkable of Sir David 

 wpvery ftrewster's discoveries, at all events, that which 

 rystals probably cost him most labour to develope, was that 

 there are crystals possessing two axes of double re- 

 fraction, and showing many remarkable phenomena, 

 which indicate a connection between optical structure 

 and crystalline form. No one before him suspected 

 the existence of a doubly-refracting structure differ- 

 ing from that so ably investigated by Huygens and 

 Malus in Iceland spar. In 1813 Sir David Brew- 

 ster had discovered coloured rings in topaz when 

 viewed by polarized light. Though intimately con- 

 nected with M. Arago's observation of colours in 

 crystallized plates, these interesting phenomena had 



a still more extraordinary and geometrical character. 

 When a plate of topaz, split by natural cleavage, was 

 presented between the polarizing and analyzing plate 

 (or rhomb of calc-spar), and at the same time 

 inclined in a certain manner, the colours were no 

 longer in broad sheets, but, if viewed closely, they 

 arranged themselves in oval rings of great beauty, 

 presenting orders of mixed colour analogous to those 

 described by Newton when formed between convex 

 glasses, and they were traversed by dark or white 

 brushes as the analyzing plate was held in the 

 dark or bright position. A second such system was 

 observed inclined at an (apparent) angle of 65 in the 

 same plate. 



Dr Wollaston afterwards (1814) discovered a phe- (530.) 

 nomenon equally beautiful in calcareous spar, of pnenomena 



which Sir D. Brewster had already perceived traces ?. f P ola " za " 



_, '. r . . tion shown 



in some other crystals. Concentric with the posi- by uniaxal 



tion of the axis of double refraction (or optic axis), crystals. 

 in a crystal of that mineral cut with two parallel 

 faces perpendicular thereto, a series of perfectly sym- 

 metric and exquisitely coloured rings are seen in po- 

 larized light, having a white or black cross travers- 

 ing them, according to the position of the analyzing 

 plate. This magnificent phenomenon, which (except- 

 ing, of course, the rings of biaxal crystals mentioned 

 above) has perhaps no parallel in optical science, is 

 seen in the most perfect manner possible in an appa- 

 ratus constructed solely of Iceland spar, cemented by 

 Canada balsam. A Nicol's single-image prism 1 is 

 used to polarize, another to analyze the light, and 

 between them is a plate of calcareous spar properly 

 cut. It is a truly astonishing paradox to see the 

 union of three perfectly transparent and colourless 

 crystals display by their union such an exquisite 

 combination of form and colour. The pole or centre of 

 the rings in calc-spar coinciding with the axis of double 

 refraction, of necessity suggested the idea that topaz, 

 which shows two systems of rings arranged round two 

 poles, must possess two axes of double refraction ; 

 in other words, that there must exist within the 

 crystal two directions (not mere Zwes), parallel to 

 which a transmitted ray emerges without subdivision 

 into two pencils. 



This probable conjecture was verified by careful (531.) 

 observation, not only in topaz, but also in a vast Double sys- 



variety of other crystals which were found bv Sir tems ? to * 



J J paz, nitre, 



David Brewster much more commonly to possess two &c< 



than one system of rings. Amongst the earliest ex- 

 amples which he observed were nitre, mica, acetate of 

 lead, and Rochelle salt. Of these the first is exceed- 

 ingly remarkable for the small inclination of the axes 

 (about 5) which permits both systems of rings to be 

 readily observed at once. It was not, however, for 

 some years (1817) that he reduced these most 



1 The invention of a most ingenious person, the late Mr William Nicol of Edinburgh. One of the doubly-refracted rays is 

 thrown completely out of the field by undergoing total reflection at the surface of a film of Canada balsam in contact with the 

 spar, whilst the other less refrangible ray proceeds quite isolated, and with scarcely any loss of brilliancy. It is of almost 

 universal application in this branch of Optics. 



