ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
813 
screw-thread i with a nut s above, which is filed square to receive the 
detachable milled head Jc. The base of this brass cone serves as face 
for attachment of crystal, and the cone can be rotated within the 
cylinder and clamped firmly to it when in any position. In order to 
prevent the cone from rotating whilst it is being clamped by the nut s, 
a steel ring m (fig. 99) is firmly attached to the cone just below the 
screw-thread by tightening the small set-screw n. This steel ring has 
two lateral lugs, which fit into the slots g before mentioned of the steel 
cylinder, and thus prevent the cone from rotating. The brass plate a 
(fig. 96) bears three small circular steel discs I, II, III in the figure, 
which are destined to receive the feet of the levelling tripod, and are so 
arranged as to occupy the angles of an isosceles triangle right-angled 
at II. 
(2) Levelling tripod. — A circular spirit-level D (fig. 100) is provided 
with two arms, each of which is bored and threaded at one end to receive 
a micrometer-screw r x and y respectively. Immediately beneath the centre 
of the spirit-level is a third but non-adjustable leg, seen in fig. 95 above 
disc II. The legs occupy the angles of an isosceles triangle right- 
angled at II as before described. On turning the screws x or y the level 
is inclined about the point o, i. e. the extremity of the middle leg of the 
levelling tripod. The amount of inclination 8 in each “ screw-plane,” 
i. e. the plane passing through o (above II) and either of the screw-axes, 
can be found if the pitch of the screw (A), the distance of the screw from 
the centre of rotation (A), and the number of rotations ( n ) are known, 
for 
For practical reasons the inclination in the screw-plane is not measured 
in degrees on a divided circle, but by means of the tangent of the angle 
expressed in the number of rotations of a screw. For small angles, the 
angle and its tangent are interchangeable, e. g. if 
n rotations = 1° 0' • 0, 
then 
5 n = 4° 59' *3 
10 n = 9° 54' * 1. 
The distance of the screws from the centre of rotation of the tripod and 
the pitch of the screws are so arranged that one rotation of the screw 
corresponds approximately to 1° of inclination in the screw plane. Each 
screw bears a divided head, permitting of 1/12° being read off and a 
single minute being estimated, whilst complete degrees (rotations) are 
indicated by the vertical scale t. 
(3) Levelled glass plate. — This consists simply of a plate of mirror 
plate-glass, 6-8 in. square, supported on the top of a wooden bracket 
firmly fixed to the wall. Three brass screws with milled heads support 
the plate, and in conjunction with the spirit-level permit of its being 
accurately levelled. 
(4) Method of preparing the apparatus for use. — The use of the discs 
I, II, III (fig. 95) is to permit the levelling tripod to be brought into a 
perfectly definite position on the top of the grinding tripod without 
1890. 3 L 
