ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
235 
the first moves, though not quite freely. The first socket is provided 
with a screw-thread, upon which works a collar fastened to the second 
socket by pins. By turning the collar the first socket is slowly raised 
or lowered as the adjustment requires. Two spiral springs on the 
second socket, fixed below, and having their upper ends in contact with 
a long plate attached to the first socket, prevent backlash between the 
screws. (Later this was effected by means of a single screw attached 
to the cross-piece of the holder.) 
Another peculiarity of the instrument is the stage, which is not con- 
nected with the body-tube, but is carried by a special foot. In the 
middle of the base-plate is let in a conically-turned toothed ring, which 
can be rotated by a small toothed wheel. On this ring is screwed a 
divided circle, which is itself surmounted by a cast-iron plate of smaller 
diameter, provided with two parallel slits, in which slide the two sides 
(tapered below) of the horse-shoe which forms the foot of the stage. 
This motion is effected and measured by means of a micrometer-screw 
provided with a divided head. The riugas well as the foot are provided 
on their under side with screws and caoutchouc rings for avoiding back- 
lash. The foot carries a pillar, which supports a plate a, bored through, 
and having on its upper side a conical projection. On this cone rotates 
a second plate, provided with a slit in which slides a metal piece, 
bored through and tapered below, to which are attached two short pins, 
which support the thin circular disc forming the stage proper. The 
form and size of the stage varies according to the experiment. When a 
high temperature is required the lenses of the Microscope are protected 
by a thick copper diaphragm, provided with a small hole. In certain 
cases this is made hollow and kept cool by a stream of cold water 
flowing through it. In fig. 20, a, b are the holes through which pass 
the tubes conveying the water. 
The burner K for heating the object, and the rod carrying the screen 
of glass or mica for moderating the temperature, are attached at right 
angles to a hollow metal column, which communicates by a branch 
tube below with the gas supply, and contains a smaller tube, reaching 
as far as the attachment of the burner, which conveys from a gas-holder 
the air necessary for the non-luminosity of the flame. The gas supply 
is cut off from the burner by a screw stopper at the top of the column. 
The latter is not screwed into the base-j^late, but fits conically into it, 
and is fastened by a nut and caoutchouc ring only so firmly that it 
may be easily turned by the hand about its axis. The contact of two 
arresting pins during the rotation automatically effects the correct 
adjustment of the burner exactly beneath the opening of the stage. 
Polarization can be effected in several ways, either by a bundle of 
glass plates Q (fig. 21) reflecting to a condensing lens the light from 
the gas-lamp X, the smoke from which passes off by the chimney Y ; or 
by means of a concave mirror ; or finally by a bundle of plates, 
wLich receives its light from an adjustable plane mirror. The 
analysing nicol is carried by an arm which slides on a vertical pillar, 
and can be clamped in any position above the eye-piece. The pillar 
ia the lower part of its length is hollow, and forms a tube which at 
about the middle projects outwards at right angles, and is then bent 
downwards towards the stage with gradually diminishing section. This 
