ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
707 
That part of it which passes through the block e , carries the micrometer 
screw. Tlio block e contains the nut in which the screw engages, and 
is therefore moved forward at each revolution of the axis. The nut is 
made up of three strips, two of which are fixed, while the third (in 
the figure facing the observer) is adjustable by the screws / /, so 
that the whole can be tightened up when the micrometer screw be- 
comes worn after long use. The block e slides on two rails g g 
screwed to the base-plate of the instrument. It carries above a plate h, 
on which the knife-holder is fastened by the screw i. The knife can be 
Fig. 104. 
adjusted to any inclination desired by means of the three screws on the 
holder. 
The toothed- wheel Tc , fixed to the axis on the right, has 500 teeth, 
and the pitch of the micrometer screw is 0 ■ 5 mm., so that by a turn of 
one tooth the knife is advanced 1 micron. 
The plate m to which the object-holder is attached by a ball-and- 
socket joint slides in swallow-tail grooves in the vertical holder l. The 
clamping ringp of the ball-and-socket joint is not in direct contact with 
the ball, but acts upon a second ring between the two, which is only 
movable in the horizontal direction, so that a horizontal pressure is 
alone produced and the adjustment of the object is not altered on 
clamping up. 
The movements of the various parts are effected in the following 
way. The axis q beneath the instrument is turned by the handle of the 
1893. 3 C 
