266 THE MIcROSCOPE. 
tubes, the pressure tube just penetrating the cover and the delivery 
tube extending nearly to the bottom of the jar; (Fig. 1) where the 
glass tubes penetrate the cover they are surrounded by rubber tub- 
ing to render the joints air tight. The pressure is obtained by the 
use of an atomizer bulb, or, in order that it may be constant, two 
bulbs are used, the second one being covered with a net to prevent 
undue distention. The bulbs used with ether-freezing microtomes 
or with some form of thermo-cautery are good, but two thick 
walled atomizer bulbs are just as efficient. 
The delivery tube and the cannula are of glass, only enough 
rubber tubing being used to make the delivery tube outside the 
jar flexible. While this jar was designed for special liquids, it has 
been found excellent for making fine injections with gelatin mass. 
With two bulbs, as in the figure, a pressure of 40 mm. of mer- 
cury may be obtained; this is sufficient for most purposes. While 
water or mercury might be used to obtain the pressure as in the 
various forms of constant pressure apparatus, the atomizer bulbs 
are preferred, as it is easier for the operator to control the pres- 
sure and adapt it to the individual cases. 
me 2. CENTERING CARD. : 
2. CENTERING CARD. In mounting objects in balsam or in 
glycerin without a cell it is somewhat difficult to get the speci- 
men in the center of the slide. It is very easy, however, if the 
slide is placed upon a card like that shown in the figure (fig. 2). 
The card is prepared by making upon it several concentric circles, 
and then cementing to it pieces of glass or Bristol board, so that 
when the slide is placed in position the center will be over the cen- 
ter of the circles. This device does not enable one to increase the 
excellence of a preparation, but it does enable one to render the 
