AGf~ICULTURAL RESEARCH, PUSA.

42 PRACTICAL BACTERIOLOGY
a rack and pinion or a spiral screw. 'rhe substage is
fitted with a substage condenser, attached to which lis
the iris diaphragm. Fitted to the end of the tail-piece
is a rnir1'01', mounted 'on a gimbal fitting. The plane
side of the mirror is 1 employed when a condenser is
used; the concave side is used only in the absence of
a condenser, and the focal length is such that light
comes to a focus on the objcct examined.
The draw-tube should be adjusted to the tube length
(vide p. 441) for which the objective is corrected; for
the majority of microscopes this is usually 160 mm.,
but as thc revolving nose-piece has a length of 18 mm.
the draw-tube should be extended only to the 142 Imu.
mark. Leitz objectives, however, are corrected for
a tube length of 170 mm. It is essential that an
objective should be used at its proper tube length,
particularly the apochromatic objectives (vide infra),
if the maximum resolution is to be obtained.
The working of the fine adjustment varies acc:ording to
the make of the microscope, for din'crcnt manufacturers have
thcir own particular type of mechanism. The older forms were
actuated by means of a milled head mounted on a pillar behind
the coarse adjustment, which turned a screw with a fine thread.
Newer models have a milled head pamllel to the coarse adjustment.
'rhe movements of the milled head raise or lowe!' the
tube in the same direction as 'the coarse adjustment. The
milled head is graduated in ,,'ifiths, and one division corrcsponds
usually to a movement of the tube of 0'002 mm. The
manner of securing a fine movement is by a system' of either
levers, c.o