Below the stage is the
ingenious 'Iris Diaphragm.' The new concentric stage
can be added at a moderate cost to the first-class
stands on the old pattern beneath the stage in either
form is a continuation of the gun-metal 'limb' which
carries the body; and this is ploughed out into a
groove for the reception of a sliding-bar, which
carries what may be termed the Secondary Body- namely,
a short tube (seen beneath the stage) capable of being
moved up and down by a milled-head, which answers the
same purpose as the secondary stage of Ross's
Microscope being made to work in a groove which is in
perfect correspondence with that wherein the principal
body works (this correspondence being secured by the
action of the planning-machine that ploughs both
grooves), the secondary body always has its axis so
perfectly continuous with that of its principal, that
no special adjustment is needed to 'centre' the greater
part of the Illuminating apparatus. The 'secondary
body' or 'cylindrical fitting' is so constructed as to
carry the Achromatic Condenser at its upper end, the
Polarizing prism at its lower, and the Selenite plates
between the two; it has not, however, any rotatory
movement of its own; but its fittings may be turned in
the tube which carries them. The Mirror is hung in the
usual way between two centers; but the semicircle that
carries these, instead of being at once pivoted to the
tube which slides upon the cylindrical stem, is
attached to an intermediate arm; and by means of this
it may be placed in such a position as to reflect light
very obliquely upon the object. Though the mode in
which the body is supported has the disadvantage of
separating the focal adjustments from each other and
from the stage-motions more widely than is the case in
the three preceding instruments, yet the difference is
scarcely perceptible in practice. The milled-heads
acting on the former are both of them in positions
in which they are easily reached by the left hand, when
the elbow is resting on the table; whilst the right
hand finds the milled-heads of the traversing stage and
of the secondary body in close proximity to each
other.