*88 Dr. Brewster on the multiplication of images , and the 
ment upon this principle is shown in Fig. 10, where ABCD is 
a tube one or two inches long, S a piece of black glass form- 
ing an angle of 33 0 with the axis of the tube; n a convex 
eye glass placed next the eye ; 0 an aperture of a circular 
or any other form in the focus of the lens 0; m a flat piece 
of topaz or rock crystal* not much larger in diameter than 
the pupil of the eye, and cut in the proper direction from the 
crystal ; BD a prism of nearly the same diameter formed out 
of rock crystal in the manner long ago described by M. Ro- 
CHONj-f so as to produce the greatest separation of the images, 
or what is still simpler, a prism of calcareous spar having the 
refraction and dispersion as much as possible corrected by 
an opposite prism of balsam of Tolu or indurated Canada 
balsam. When the instrument is thus fitted up, the rays 
RS, polarised by reflection from the glass S, are arranged into 
their complementary colours by the crystallized plate m, and 
are afterwards separated into two distinct pencils by the dou- 
ble refraction of the prism BD. An eye, therefore, placed at 
n, will see two distinct images of the aperture 0, and the 
colour of the one image will be complementary to that of the 
other. These images will exhibit alternate variations of colour 
by turning round either the tube, or the polarising plane S. If 
* A thin film of sulphate oi lime is much better than any other mineral, as it re- 
quires no trouble to prepare it. Topaz is preferable to rock crystal, as the latter very 
often gives false tints, from a want of uniformity of structure. Mr. Sanderson, an 
ingenious lapidary in Edinburgh, has cut about twelve plates of rock crystal parallel 
to the axis of the pyramid, and observed, that all of them were filled with veins and 
imperfections radiating from the axis. In a large pyramid about 2j inches in dia- 
meter, these radiations are arranged in the form of a cross, forming angles of 6o # and 
1 20®, and they terminate on the faces of the pyramid. 
f Journal de Physique, 1801. Memoire sur It Micrometre de Cristal dt Roche. 
Paris, 1807. 
