ARRANGEMENT FOR TRANSPARENT OBJECTS. 153 



so that a slight pressure is needed to flatten or extend it, recourse 

 may be had to the use of the Aquatic Box (§ 97) or of the Com- 

 pressorium (§ 99), without the introduction of any liquid between 

 the surfaces of glass. In a very large proportion of cases, how- 

 ever, either the objects to be examined are already floating in 

 fluid, or it is preferable to examine them in fluid on account of 

 the greater distinctness with which they may be seen. If such 

 objects be minute, and the quantity of liquid be small, the drop is 

 simply to be laid on a slip of glass, and covered with a plate of 

 thin glass ; if the object or the quantity of liquid be larger, it 

 will be better to place it in the Aquatic Box ; whilst, if the object 

 have dimensions which render even this inconvenient, the Zoophyte 

 Trough (§98) will afford the best medium for its examination. In 

 the case of living animals, whose movements it is desired to limit 

 (so as to keep them within the field of view) without restrain- 

 ing them by compression, the Author has found the following 

 plan extremely convenient. The drop of water taken up with 

 the animal by the Dipping-tube being allowed to fall into a concave 

 slide (Fig. 104), the whole of the superfluous water may be removed 

 by the Syringe (§ 101), only just as much being left as will keep 

 the animal alive. If the animal be very minute, it is convenient 

 to effect this withdrawal by placing the slide on the stage of the 

 Dissecting Microscoxje (§ 37), and by working the syringe under 

 the magnifier; and it will be found, after a little practice, that the 

 complete command which the operator has over the movements of 

 the piston, as well as over the place of the point of the syringe, 

 enables him to remove every drop of superfluous water without 

 drawing the animal into the syringe. "When, on the other hand, 

 it is desired to isolate a particular animal from a number of others, 

 the syringe may be conveniently used, after the same fashion, to 

 draw it up and transfer it to another slide ; care being, of course, 

 taken that the syringe so employed has a sufficient aperture to 

 receive it freely. If it be wished to have recourse to compression, 

 for the expansion or flattening of the object, this may be made 

 upon the ordinary slide, by pressing down the thin-glass cover with 

 a pointed stick ; and this method, which allows the pressure to be 

 applied where it may chance to be most required, will generally be 

 found preferable for delicate portions of tissue which are easily 

 spread out, and which, in fact, require little other compression than 

 is afforded by the weight of the glass cover, and by the capillary 

 attraction which draws it into proximity with the slide beneath. 

 A firmer and more enduring pressure may be exerted by the 

 dexterous management of a well-constructed Aquatic Box; and 

 this method is peculiarly valuable for confining the movements of 

 minute animals, so as to keep them at rest under the field of the 

 microscope, without killing them. It is where a firm but graduated 

 pressure is required, for the flattening-out of the bodies of thin 

 semi-transparent animals, without the necessity of removing them 



