54 
MR. HORNER AND SIR DAVID BREWSTER ON 
and the other at C. These images B, C, separate by the inclination of the plate 
exactly like those in fig. 1., and all the four, A, B, C, and D, have the same absolute 
and relative polarization as the four analogous images seen through the new sub- 
stance, with this difference only, that none of them are nebulous. 
If we conceive the vein E F to consist, as in fig. 4., Fig- 4. 
of a great number of small crystals, a, b, c, d, &c. in A — 
place of one, the very same effects will be produced. X b 
When we look through the new substance, the / 
qZ. 
multiplication of images takes place in whatever azi- 
muth we incline the plate, the elongated images being always perpendicular to the 
azimuth of inclination. Hence it follows, that these images are produced by numbers 
of minute crystals lying in or near the azimuth in which the plate is inclined; and 
that these crystals have their axes all inclined to that of the plate which contains 
them, at the same angle as the vein E F, figg. 2. & 4., is inclined to the axis of the 
rhombohedron of Iceland spar. But the remarkable result of these observations is, 
that in place of one set of crystals, or sometimes three sets, which occur in calcareous 
spar in three different azimuths, we have here an infinite number of them lying in 
every possible azimuth, and these so small in their dimensions that they cannot be 
recognised by the most powerful microscopes, except as dark specks disseminated 
through the general mass ; and yet they indicate by their action on light, not only 
their existence, but the position of their axes, and their doubly refracting and polar- 
izing structure, as unequivocally as if we could handle them, and cleave them, and 
place them upon the goniometer. 
It may now be asked why the images are nebulous, and not distinct as in calca- 
reous spar. The reason is, that the substance is imperfectly crystallized like the 
agate, mother-of-pearl and other bodies in which the doubly refracting force separates 
the incident light into two oppositely polarized pencils, which are not perfectly equal 
and similar, but which differ from each other, sometimes in the intensity of their 
light, sometimes in the distinctness of the image, sometimes in the nature or bright- 
ness of the colour, and sometimes in more than one of these characters. But though 
the new substance resembles the crystals above mentioned in giving dissimilar pencils 
of doubly refracted light, it stands unique among all bodies with which I am 
acquainted in possessing the extraordinary system of composite crystallization, in 
which an infinite number of crystals are disseminated equally in every possible azi- 
muth through a larger crystalline plate, having their axes all inclined at the same 
angle to that of the larger plate, and producing similar phenomena in every direction, 
and through every portion of the plate ; or we may describe this remarkable structure 
by saying that the minute elementary crystals form the surfaces of an infinite number 
of cones whose axes pass perpendicularly through every point of the larger plate*. 
A rude idea of this structure is given by the beautiful cones, or rather pyramids of microscopic crystals of 
titanium which I have somewhere described as existing within the pyramids of many crystals of amethyst from 
the Brazils. 
