et, which are usually shod
with cork; in the second, by the dead weight of the foot-plate. The
tripod is mechanically the more correct form, and for practical use is
much to be preferred. Its chief rival, the Jackson foot (Fig. 41, c),
is based upon the same principle, and on the score of appearance has
much to recommend it.

~2.~ The ~body tube~ (Fig. 40, b) may be either that known as the "long"
or "English" (length 250 mm.), or the "short" or "Continental" (length
160 mm.). Neither length appears to possess any material advantage over
the other, but it is absolutely necessary to secure objectives which
have been manufactured for the particular tube length chosen. In the
high-class microscope of the present day the body tube is usually
shorter than the Continental, but is provided with a draw tube which,
when fully extended, gives a tube length greater than the English, thus
permitting the use of either form of objective.

[Illustration: FIG. 42.--Coarse adjustment.]

[Illustration: FIG. 43.--Fine adjustment.]

For practical purposes the tube length = distance from the
end of the nosepiece to the eyeglass of the ocular. This is
the measurement referred to in speaking of "long" or "short"
tube.

~3.~ The ~coarse adjustment~ (Fig. 40, c) should be a rack-and-pinion
movement, steadiness and smoothness of action being secured by means of
accurately fitting dovetailed bearings and perfect correspondence
between the teeth of the rack and the leaves of the pinion (Fig. 42).
Also provision should be made for taking up the "slack" (as by the
screws _AA_, Fig. 42).

~4.~ The ~fine adjustment~ (Fig. 40, d) should on no account depend upon
the direct action of springs, but should be of the lever pattern,
preferably the Nelson (Fig. 43). In this form the unequal length of the
arms of the lever secures very delicate movement, and, moreover, only a
small portion of the weight of the body tube is transmitted to the
thread of the vertical screw actuating the movement.

[Illustration: FIG. 44.--Spindle head to fine adjustment.]

A spindle milled head (Fig. 44) will be found a very useful device to
have fitted in place of the ordinary milled head controlling the fine
adjustment. In this contrivance the axis of the milled head is prolonged
upward in a short column, the diameter of which is one-sixth of that of
the head. The spindle can be rapidly rotated between the fingers for
medium power adjustments whi