AQUATIC BOX OR ANIMALCULE CAGE. 149 



it be desired to prevent the included fluid from evaporating. 

 But as it is desirable that this glass should be thin enough to 

 allow a l-4th-inch objective to be employed for the examination 

 of Animalcules, &c., and as such thin glass is extremely apt to 

 be broken, it is a much better plan to furnish the brass cover 

 with a screw-cap, which holds the brass disk with sufficient 

 firmness, but permits it to be readily replaced when broken ; 

 and as the looseness of this fitting gives ample space for the 

 escape of air or fluid around the margin of the disk, no special 

 aperture is needed. It is always desirable, if possible, to prevent 

 the liquid from spreading to the edge of the disk ; since any 

 objects it may contain are very apt, in such a case, to be lost 

 under the opaque ring of the cover ; this is to be avoided by 

 limiting the quantity of the liquid introduced, by laying it upon 

 the centre of the lower plate, and by pressing down the cover 

 with great caution, so as to flatten the drop equally on all sides, 

 stopping short when it is spreading too close to the margin. 

 With a little practice, this object may in general be successfully 

 attained ; but if so much superfluous liquid should have been in- 

 troduced, that it has flooded the circumference of the inclosed 

 space, and exuded around the edge of the disk, it is better to 

 wipe the whole perfectly dry, and then to introduce a fresh drop, 

 taking more care to limit its quantity and to restrain it within 

 convenient bounds. If the box be well constructed, and the 

 glass disks be flat, they will come into such close contact, that 

 objects of extreme thinness may be compressed between them; 

 hence not only may such small animals as Water-fleas (Entomo- 

 straca) be restrained from the active movements which preclude 

 any careful observation of their structure, and this without any 

 permanent injury being inflicted upon them, but much smaller 

 creatures, such as Wheel-Animalcules (Rotifera), or Bryozoa, 

 may be flattened out, so as to display their internal organization 

 more clearly, and even the larger Infusoria may be treated in 

 like manner. The working Microscopist will find it of great 

 advantage to possess several of these aquatic boxes, of different 

 sizes ; and one or two of them may have the glass cover of 

 stronger glass than the rest, and firmly fixed in its rim, so that, 

 if the cover be made to slide equably on the box, the instrument 

 (in hands accustomed to careful manipulation) may be made to 

 answer the purpose of a compressorium ( 70). 



69. Zoophyte Trough. -For the examination of living aquatic 

 objects, too large to be conveniently received into the Aquatic 

 Box, the Zoophyte trough contrived by Mr. Lister may be em- 

 ployed with great advantage. This consists of a trough of the 

 shape represented in Fig. 48, formed of plates and slips of plate- 

 glass cemented together by marine glue ; of a loose vertical plate 

 of glass, just so much smaller than the front or back of the in- 

 side of the trough, as to be able to move freely between its sides; 

 and of a horizontal slip of glass, whose length equals that of the 



