ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
253 
we have 
^ _ m sin (f> 
v 
To avoid calculations during the observation, Dr. Wellmann has 
prepared tables of the value of cl for different values of and v. 
Til 
— is usually a very small quantity, which for the prism and lenses 
used by the author amounted only to 9 /x. Accordingly, to pass from 0 /x 
to 9 /x the prism must be turned 
through 90°, so that great exactness 
is obtained for even a comparatively 
rough reading. Thus a reading of 
1/10 degree on the circle gives an 
exactness of 
9 
9IFT0 = °’ 01 ^ 
The apparatus itself, as con- 
structed by Schmidt and Hansch, 
consists of two parts (fig. 15 A 
and B). The divided circle k is, 
by means of the socket h, passed 
over the body-tube of the Micro- 
scope, and is fastened by three 
screws. Only two opposite quad- 
rants are used, and the rest of the 
circle, together with the middle por- 
tion of the quadrants, is cut away to 
diminish the weight of the appa- 
ratus. The eye-piece is then inserted 
in the body-tube. Above the socket 
of the divided circle, which projects upwards, another is fitted which 
easily turns about it. This carries in its upper part the prism p of 
rock-crystal, with refracting angle of 70°. Beneath are two projecting 
arms, one of which n serves as vernier reading to 1/10 degree, and the 
other b to balance and turn the apparatus. 
Polarizing Prisms.*— Dr. W. Grosse calls attention to the im- 
portant part played by calc-spar prisms in so many physical instruments, 
and regrets the high price of the material and the great loss which 
takes place in the course of preparation of the prisms. The various 
forms of prism are classified as follows : — 
I. Prisms in which both rays wholly or partially occupy the field of 
view. Besides the older well-known forms of Wollaston, Senarmont, and 
Rochon, there are the more recent prisms of Dove and Abbe, the latter 
of which consists of an equilateral prism of calc-spar, with wedges of 
crown glass on the sides. 
II. Prisms in which the central zone of the field of view is occupied 
only by one ray (the extraordinary). 
* Zeitschr. f. Instrumentenk., x. (1800) p. 445. 
Fig. 15. 
