ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
585 
mcnts which are communicated to the latter by means of the divided 
head i. Since the volume of the air in the cylinder g depends not only 
on the temperature but also on the pressure, it is necessary in order to 
keep a constant temperature that the level of the water in the reservoir 
should be kept fairly constant. 
The preparation beneath the hot-stage will have a lower temperature 
than the stage itself. This difference must be determined for different 
temperatures of the stage by means of a thermometer occupying the 
position of the preparation. At 30° the difference amounted to 1°*5, 
and at 60° to 3°. 
For the perfect air-tight closing of the cylinder g, the piston must 
be oiled. This oiling can be effected through the neck h after removing 
the cap b (fig. 87) and unscrewing the screw s. For working at a tem- 
perature of 50° the reclosing of h by the screw s can be done at the 
ordinary temperature, but for work at a temperature of 60° the cylinder 
must first be cooled to -f 10° before closing h. 
Auxiliary Apparatus for the Adjustment with Immersion Objec- 
tives.* — Herr A. van Delden describes a simple arrangement which in 
difficult cases renders the adjustment with immersion objectives quick 
and safe. 
Round the outer tube T 2 (fig. 89) is fastened by means of the three 
screws s 2 , and s 3 , a strong ring which rests above against the edge 
of the spring socket in which 
the inner tube slides. As seen 
in the figure, the ring is cut 
through to allow room for the 
rack B. Through the broader 
end thus obtained passes a mi- 
crometer screw m which is pro- 
vided with a divided head and 
long vertical index I (fig. 90). 
The head has a diameter of 
about 1 cm., and is divided into 
20 divisions. The pitch of the 
screw is 0 • 5 mm., and the num- 
ber of revolutions can be read 
directly on the vertical index 
which is divided into half milli- 
metres. 
By estimating the tenth of a 
division on the head, differences 
up to 0* 0025 mm. can be mea- 
sured. The micrometer screw 
moves parallel to the rack B, 
and presses against the column 
A which is moved by the micrometer screw of the stand (figs. 90 and 
91). The apparatus is used in the following way : — Adjustment is first 
made on a clearly coloured preparation with a fairly strong dry system 
(e. g. Zeiss I) to F), the screw m is then turned until it rests on the 
* Zeitschr. f. wiss. Mikr., xii. (1835) pp. 15-18 (3 figs.). 
Fig. 89. 
