e of the microscope (Fig. 40 f), and must be accurately
adjusted so that its optical axis coincides with that of the objective.
Vertical movement of the entire substage apparatus effected by means of
a rack and pinion is a decided advantage, and some means should be
provided for temporarily removing the condenser from the optical axis of
the microscope.

[Illustration: FIG. 47--Optical part of Abbe illuminator.]

With the oil immersion objective, however, an ~achromatic condenser~,
giving an illuminating cone of about 0.9, should be used if the full
value of the lens is to be obtained. It is generally assumed that a good
objective requires an illuminating cone equivalent to two-thirds of its
numerical aperture. The best Abbe condenser transmits a cone of about
.45 whilst the aperture of the 1/12 inch immersion lenses of different
makers varies from 1.0 to 1.4, hence, the efficiency of these lenses is
much curtailed if the condenser is merely the Abbe. These improved
condensers must be absolutely centered to the objective and capable of
very accurate focussing otherwise much of their value is lost.

~8. Mirrors.~--Below the substage condenser is attached a gymbal carrying
a reversible circular frame with a plane mirror on one side and a
concave mirror on the other (Fig. 40, g). The plane mirror is that
usually employed, but occasionally, as for example when using low powers
and with the condenser racked down and thrown out of the optical axis,
the concave mirror is used.

~9. Oculars, or Eyepieces.~--Those known as the Huyghenian oculars (Fig.
48) will be sufficient for all ordinary work without resorting to the
more expensive "compensation" oculars. Two or three, magnifying the
"real" image (formed by the objective) four, six, or eight times
respectively, form a useful equipment.

As an accessory ~Ehrlich's Eyepiece~ is a very useful piece of apparatus
when the enumeration of cells or bacteria has to be carried out. This is
an ordinary eyepiece fitted with an adjustable square diaphragm operated
by a lever projecting from the side of the mount. Three notches are made
in one of the sides of the square and by moving the lever square
aperture can be reduced to three-quarters, one-half or one-quarter of
the original size.

~10. Objectives.~--Three objectives are necessary: one for low-power
work--e. g., 1 inch, 2/3 inch, or 1/2 inch; one for high-power
work--e. g., 1/12 inch oil immersion lens; and an intermediate
"m