ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
387 
turned aside. The arm moves parallel to the glass tablet, so that very 
small free-swimming animals can readily be caught and held fast 
between the two glasses. The thin cover-glass is cut off at the top, thus 
allowing reagents to be added to the drop of water while the animals are 
under examination ; it is much larger than the glass tablet and therefore 
allows the highest powers to be used all over the field and to the very 
edges of the glass tablet, which is of great importance in practical work. 
In nearly all other compressors the central part of the field can alone be 
reached with high powers. Messrs. Baker are the makers. 
Air-pump for Microscopical Purposes.* — Dr. A. Koch describes on 
air-pump which has been used for many years at Gottingen, for removing 
air from microscopical preparations. 
The apparatus consists of a vertical upright B B clamped on to the 
edge of the table. The upright is drilled out, and through the cylindri- 
cal passage runs a piston at the 
lower end of which is the handle 
A. The hollow in B B is in 
communication with the cavity 
of a rectangular box D, closed by 
a glass plate, and having cavity 
only just big enough to admit a 
slide. The lever C, when placed 
vertically, closes the communi- 
cation between D and B B and 
opens another aperture which 
places B B in communication 
with the air. When placed hori- 
zontally the air space of D and 
the hollow of B B are connected. 
Then by putting down the handle 
A the air in the space D becomes 
rarefied and the bubbles are 
drawn out from underneath the 
cover- glass, as may be seen by 
means of the lens F and the 
mirror E. By alternately placing 
the lever C in the horizontal 
and vertical position and pulling 
down and pushing up the piston the preparation may be completely 
freed from air. 
Fig. 47. 
(4) Photomicrography. 
Photography of Gratings and Micrometers engraved on Glass, 
M. Izarn, struck with the fineness with which the most delicate details 
of a plate on glass can be photographically reproduced, has made ex- 
periments on the photography of gratings and micrometers engraved on 
glass, and obtained reproductions so perfect that it is impossible at first 
sight to distinguish between copy and original. Twenty years ago 
Lord Kayleigh J made experiments in the same direction, but abandoned 
* Zeitschr. f. wiss. Mikr., ix. (1893) pp. 298-9 (1 fig.). 
t Comptes Rendus, cxvi. (1893) pp. 5U6-8. J Phil. Mag., 1872 and 1874. 
2 D 2 
