IN PLATES, TUBES, AND CYLINDERS OF GLASS. 371 
and kept in this position by the wooden holdfast H; the 
brass: pieces Sac, S’a'b'c’, having been previously placed at 
such a distance from each other, that the two plates will meet 
at H, without breaking, or without any permanent change of 
form. . The apparatus ER, for observing the tint, is shewn sepa-_ 
rately in Fig..16. It consists of an eye-piece E, to which is at-- 
tached a reflector R, made of several plates of the thinnest. 
glass, about 14 long and an inch broad, and placed close to each 
other. The eye-piece E consists of two tubes, one of which is 
moveable within. the other. The moveable tube contains an 
achromatic prism, mn, of calcareous-spar, with a convex lens, 
op, about an inch in focal length, placed either above or below 
it. When.this apparatus is set upon the edge of AB, by means 
of the forked arms e, f; the reflector R is turned round, till the 
plane of reflection is cut at an-angle of 45°, by the plane of the 
plate AB, and is placed at such an.angle, that the light which it 
reflects through the edge of the plate AB, and up the tube, is 
completely polarised. The moveable tube is then turned round, 
till the tints appear on the edge of one of the images of the 
glass plate. In order to avoid the confusion arising from two 
images, the achromatic prism may be constructed in such a. 
manner that only one of the images is visible. 
3A 2 XIX 
