238 NOTES AND MEMORANDA. 



This is furnished at equal distances from its centre with two well- 

 polished shallow cavities of circular form, which are connected with 

 each other by one or more capillary channels. These channels are 

 likewise polished, and to permit of a greater field in focussing for 

 their contents, the groove of the tube is made triangular in section, 

 with one side forming a right angle with the surface 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. If held down for a moment 

 with the hands, the air within the cavities will become slightly rarefied, 

 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 cavities is occupied with a bubble 

 of air, while a thin annulus about the circumference as well as the 

 connecting capillary tubes are occupied by the fluid. The slide is 

 now ready for inspection. If placed beneath the microscope, and the 

 instrument is focussed upon the connecting channel, a number of cor- 

 puscles, red and white, will be observed, but quite quiescent. Let 

 the finger be now approached to the neighbourhood 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 direction of the current is reversed. The current is caused by 

 the expansion 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, granule, or 

 cell, 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 behaviour of the objects beneath it, tho mind is charmed 

 with the simplicity of the means by which these motions are con- 

 trolled. In the cell here described, no foreign liquid is added to the 

 material under examination. Moreover, if each cell be entirely 

 filled with liquids of different densities, the cell holding the denser 

 liquid being placed slightly uppermost upon the rotating stage of 

 the microscope, the action of gravity will cause two currents to flow 

 in opposite directions through the communicating channels, and in 

 this way the phenomena of transfusion, crystallization, &c, may be 

 observed for a considerable length of time, which otherwise are 

 brought to sight only with difficulty. We have to thank the Editor 

 of the ' Journal of the Franklin Institute ' for the block. 



