AQUATIC BOX. ZOOPHYTE TROUGH. 131 



be spread out, or the object be flattened, to tbe degree most con- 

 venient for observation. If the glass disk which forms the lid be 

 cemented or burnished into the brass ring which carries it, a small 

 hole should be left for the escape of air or superfluous fluid ; and 

 this hole may be closed up with a morsel of wax, if 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 glass 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 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 enclosed 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 (Entomostmca) 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 instniment (in hands accustomed to careful 

 manipulation) may be made to answer the purpose of a Compres- 

 sorium (§ 99). 



98. Zoophyte Trough.— "For the examination of living Aquatic 

 objects too large to be conveniently received into the Animalcule 

 cage, the Zoophyte trough, contrived by Mr. Lister, may be employed 

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



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