Cuapr. V., § 5.] 
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- 
OPTICS.—SIR DAVID BREWSTER. 
913 
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 cale-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. 
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 
Dr Wollaston afterwards (1814) discovered a phe- 530.) 
nomenon equally beautiful in calcareous spar, of Phenomena 
which Sir D, Brewster had already perceived traces ° Higarrce 
in some other crystals. Concentric with the posi- py uniaxal 
tion of the axis of double refraction (or optic avis), crystals. 
(528.) 
and in 
bodies sub- 
jected to 
pressure. 
(529.) 
Discovery 
of biaxal 
crystals. 
those of natural crystals. 
Sir David Brewster at first ascribed these effects 
to the direct effect of the heat in the glass upon 
light, and compared its simultaneous influence over 
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. 
III. Possibly the most remarkable of Sir David 
Brewster’s discoveries,—at all events, that which 
probably cost him most labour to develope,—was that 
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? 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 
therings in cale-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 
erystal two directions (not mere lines), parallel to 
which a transmitted ray emerges without subdivision 
into two pencils. 
there are crystals possessing two axes of double re- 
fraction, and showing many remarkable phenomena, 
which indicate a connection between optical structure 
This probable conjecture was verified by careful (631.) 
observation, not only in topaz, but also in a vast Double sys- 
variety of other crystals which were found by ome - 
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 
erystallized plates, these interesting phenomena had 
David Brewster much more commonly to possess two go, 
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 
universal application in this branch of Optics. 
VOL. T. 
proceeds quite isolated, and with scarcely any loss of brilliancy. It is of almost 
5z 
