le the larger milled head can be slowly
moved when focussing high powers.

~5.~ The ~stage~ (Fig. 40, e) should be square in shape and large in
area--at least 12 cm.--flat and rigid, in order to afford a safe support
for the Petri dish used for plate cultivations; and should be supplied
with spring clips (removable at will) to secure the 3 by 1 glass slides.

A mechanical stage must be classed as a necessity rather than a luxury
so far as the bacteriologist is concerned, as when working with high
powers, and especially when examining hanging-drop specimens, it is
almost impossible to execute sufficiently delicate movements with the
fingers. In selecting a mechanical stage, preference should be given to
one which forms an integral part of the instrument (Fig. 45) rather than
one which needs to be clamped on to an ordinary plain stage every time
it is required, and its traversing movements should be controlled by
stationary milled heads (Fig. 45, _AA'_). The shape of the aperture is a
not unimportant point; it should be square to allow of free movement
over the substage condenser. The mechanical stage should be tapped for
three (removable) screw studs to be used in place of the sliding bar, so
that if desired the Vernier finder (Fig. 45, _BB'_), such as is usually
fitted to this class of stage, or a Maltwood finder, may be employed.

[Illustration: FIG. 45.--Mechanical stage.]

[Illustration: FIG. 46.--Iris diaphragm.]

~6. Diaphragm.~--Separate single diaphragms must be avoided; a revolving
plate pierced with different sized apertures and secured below the stage
is preferable, but undoubtedly the best form is the "iris" diaphragm
(Fig. 46) which enters into the construction of the substage condenser.

~7.~ The ~substage condenser~ is a necessary part of the optical outfit.
Its purpose is to collect the beam of parallel rays of light reflected by
the plane mirror, by virtue of a short focus system of lenses, into a
cone of large aperture (reducible at will by means of an iris diaphragm
mounted as a part of the condenser), which can be accurately focussed on
the plane of the object. This focussing must be performed anew for each
object, on account of the variation in the thickness of the slides.

The form in most general use is that known as the Abbe (Fig. 47) and
consists of a plano-convex lens mounted above a biconvex lens. This
combination is carried in a screw-centering holder known as the substage
below the stag