ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
865 
central aperture, are screwed two brass plates P P', isolated from each 
other. Attached to the plate P' is a small bracket Z, the wedge-shaped 
pointed end of which projects slightly over the edge of the central 
aperture of the stage, and carries a small 
projecting pin. 
A second similar bracket V can be 
screwed over the first by the screw r, 
so that the pin of Z fits into a correspond- 
ing hole in l ' . 
Opposite the clamp thus formed by 
Z and l', is a similar one L L', carried 
by the plate P. This clamp, however, 
is not rigidly fixed like the first, but can 
be made to approach it to a certain ex- 
tent by pressing on the end against a 
spring attached to the under side of P. 
The two clamps serve to stretch be- 
tween them the two pieces of platinum 
foil B, which are pierced in the centre 
with small apertures. After removal of 
the upper parts L' Z', the clamps are 
pressing against the spring, and the two pieces of foil are placed 
upon the lower parts L Z, so that the projecting pins pass through corre- 
sponding holes in the foil. The upper parts are then put on, and the 
pressure on the spring is released, so that the pieces of foil are firmly 
stretched, and form a metallic connection between the plates P and P'. 
The crystal plate is inserted between the pieces of foil and the pres- 
sure due to the spring suffices to hold it firmly in position, even when 
the Microscope is directed horizontally. For greater security, however, 
the lower piece of foil is provided with a small projecting rim. The 
apparatus is attached to the Microscope-stage by the ordinary clips, 
which press upon the pins s s’ projecting from the plate of slate. The 
electric current is brought by the wires K K, which are screwed to 
the plates P, P\ In the Berlin Mineralogical Museum, the current is 
supplied by a Raub thermo-battery, which produces a current of 15 
amperes, with electromotive force of 3 volts. A rheostat is used for 
the regulation of the temperature. 
For observation with convergent polarized light at high tempera- 
tures, the lens fastenings of objective and condenser must be protected 
with capsules of horn or paper ; but even with this precaution a close 
approach of condenser and objective can only be allowed for a very 
short time. 
The apparatus is, therefore, not well adapted for observation between 
crossed nicols in convergent light, and can only be used with advantage 
for small crystals. 
The third apparatus, heated by gas, for observation with parallel and 
convergent light at high temperatures, is intended to remedy these 
defects. It differs from the two just described in so much that it is 
not connected with the stage of the Microscope, and so cannot be 
rotated with it. It is therefore most conveniently used with a Micro- 
scope such as that of Dick and Swift, in which the two nicols can be 
Fig. 96. 
brought closer together by 
