{ 1", 2/3", 1/2":
"blood film," stained with Jenner's } for { 1/6", 1/8"
or Romanowsky's stain. } { 1/12" oil

2. A thin cover-slip preparation }
of a young cultivation of } { 1/8" dry
_B. diphtheriae_ (showing } for {
segmentation) stained with } { 1/12" oil
methylene-blue. }

~Accessories.~--_Eye Shade_ (Fig. 49).--This piece of apparatus consists
of a pear-shaped piece of blackened metal or ebonite, hinged to a collar
which rotates on the upper part of the body tube of the microscope. It
can be used to shut out the image of surrounding objects from the
unoccupied eye, and when carrying out prolonged observations will be
found of real service.

_Nosepiece._--Perhaps the most useful accessory is a nosepiece to carry
two of the objectives (Fig. 50), or, better still, all three (Fig. 51).
This nosepiece, preferably constructed of aluminium, must be of the
covered-in type, consisting of a curved plate attached to the lower end
of the body tube--a circular aperture being cut to correspond to the
lumen of that tube. To the under surface of this plate is pivoted a
similarly curved plate, fitted with three tubulures, each of which
carries an objective. By rotating the lower plate each of the objectives
can be brought successively in to the optical axis of the microscope.

[Illustration: FIG. 49.--Eye shade.]

For critical work and particularly for photo-micrography, however, the
interchangeable nosepiece is by no means perfect as it is next to
impossible to secure accurate centreing of each lens in the optical
axis. For special purposes, therefore, it is necessary to employ a
special nosepiece such as that made by Zeiss or Leitz into which each
objective slides on its own carrier and upon which it is accurately
centred.

[Illustration: FIG. 50.--Double nosepiece.]

[Illustration: FIG. 51.--Triple nosepiece.]

_Warm Stage_ (Fig. 52).--This is a flat metal case containing a system
of tubes through the interior of which water of any required temperature
can be circulated. It is made to clamp on to the stage of the
microscope by the screws _A A'_, and is perforated with a large hole
coinciding with the optical axis of the microscope; a short tube B,
projecting from one end of the warm stage permits water of the desired
temperature to be conducted from a reservoir through a length of rubber
tubing to the interior of the stage and a similar