ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
549 
metrically, as in fig. 60, but in such a way (fig. 61) that the boundary 
lines fall in the plane of the double refraction. By this means the 
ordinary image A a of the one aperture is formed above the extraordinary 
image B e of the other, and no coloured seams appear on the boundary 
lines, for the simple reason that no deviation nor dispersion takes place 
in the direction at right angles to these lines. 
Fig. 60. 
Fig. 61. 
For the same calcite prism, in order to obtain the right effect, the 
apertures A B (fig. 61) must be chosen smaller or closer together than 
the apertures a b (fig. 60). But since in this new arrangement no 
achromatism (and consequent diminution of deviating effect) of the 
calcite prism is necessary, the deviation by double refraction can be 
made essentially greater than in the first case, so that apertures really 
larger and farther apart than in the ordinary apparatus can be used. 
The arrangement here described can be applied to all apparatus in 
which the juxtaposition of the ordinary and extraordinary images of two 
apertures is effected by double refraction. 
A Revolving Table.* — The following is an account of Mr. F. L. 
Morton’s device : — 
I. Saw out a circular board 18 in. in diameter, and ornament the 
edge if you choose. On the under side, and about 2 in. from the edge, 
place four castors equidistant, and have the board rest firmly on them. 
In the centre of the under side bore a hole nearly through the board 
and insert a piece of brass tube. Stain or paint the board at pleasure. 
II. Cut a piece of thick pasteboard 18 in. square and to the centre 
of it fasten a block 2 x 2 in. by 1 in. thick, letting a brass pin 
stick up an inch from its centre, having it of such size as to work 
loosely in the tube. 
II L Place the second apparatus on a small stand or table, adjust 
the first apparatus over it so that the pin will fit into the tube, revolve 
the top part upon the pin as a centre guide and upon the castors as 
lateral supports. If the castors are noisy make a track with felt for 
them to run on. 
* Amer. Mon. Micr. Journ., xiii. (1892) p. 120. 
