IN PLATES, TUBES, AND CYLINDERS OF GLASS. 371 



and kept in this position by the wooden holdfast H; the 

 brass pieces Sabc> 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 1^ 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, m n, 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,/, 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. 



3A2 XIX 



