MICROSCOPY. 877 
focussing for their contents, the groove of the tube is made trian- 
gular in section, with one side forming a right angle with the sur- 
face of the slide, and the other forming with it a very large angle. 
The arrangement of the cell, or moist chamber, is as follows: In 
order that the current shall be most sensitive, the slide should 
first be brought nearly to the temperature of the body by holding 
it for a few minutes in the hand. A small quantity of the liquid 
to be examined (blood, for example), is then to be placed in each 
cell, and a thin cover glass placed upon them. held down for 
a moment with the hands, the air within the cavities will become 
slightly rarified, and the cover glass so firmly held in place by 
atmospheric pressure as to require no artificial attachment. Upon 
removal of the fingers, it will be found that the centre of the cay- 
ities is occupied with a bubble of air, while a thin annulus abou 
the circumference, as well as the connecting capillary tubes, is 
Fig. 75. 
occupied by the fluid. The slide is now ready for inspection. If 
Placed beneath the microscope, and the instrument is focussed 
“pon the connecting channel, a number of corpuscles, red and 
white, will be observed, but quite quiescent. Let the finger be 
now approached to the neighborhood of either cell, when at once a 
current, more or less rapid, according to its proximity, commences 
to flow beneath the object glass; remove the finger, and the direc- 
tion of the current is reversed. The current is caused by the 
€xpansion of the air bubble in the cell, in consequence of the heat 
radiated from the finger; and its rapidity may be controlled to a 
nicety. by regulating the proximity of the finger. So sensitive is 
the apparatus, that even with the highest powers, a corpuscle, 
Stanule or ceil in the field of view, may be leisurely turned over and 
Over in any desirable position, thus affording an unequalled means 
of observation and study to the microscopist ; and while the eye 
IS examining at leisure the behavior of the objects. beneath it, the 
