124 



THE MICROSCOPE AND ITS REVELATIONS. 



closed up with a morsel of wax, if it be desired to prevent the included 

 fluid from evaporating. But as it is desirable that the cover-glass should 

 be thin enough to allow a l-4th or a l-6th inch Objective to be em- 

 ployed, 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. It is 



always desirable, if possible, to pre- 

 s ' IGi D & vent 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 limit- 

 ing the quantity of liquid introduced, 

 by laying it upon the centre of the 

 lower plate, and by pressing dovn 

 the cover with great caution, so as to 

 flatten the drop equally on all side , 

 stopping short when it is spreading 

 too close to the margin. If the Live- 

 box be well constructed, and the glass 

 disks be quite flat, they will come in- 

 to such close contact, that objects of 

 extreme thinness may be compressed 

 between them; and it may thus be 

 made, with a little practice, to serve 

 the purpose of a Compressor ( 125). 

 In its ordinary form, however, the elevation of the object- tablet above 

 the stage prevents the Live-box from being used with the Achromatic 

 Oon denser or Paraboloid: but another form is made by Mr. Swift, in 

 which the object-tablet is fixed at the bottom of the tube, flush with the 

 surface of the plate (as shown at c); and as the covering disk is fixed to 

 the bottom of the cover-tube, and thus slides inside the box-tube, the ob- 

 ject can be illuminated by any of the means applicable to objects con- 

 tained in ordinary flat cells ( 123). The only disadvantage of this con- 

 struction is that the cover-disk must be fixed in the tube which carries it. 

 123. Infusoria, minute Algae, etc., however, can be well seen by plac- 

 ing a drop of the water containing them on an ordinary slide, and laying a 

 thin piece of covering-glass on the top. And objects of somewhat greater 

 thickness can be examined by placing a loop or ring of fine cotton-thread 

 upon an ordinary slide, to keep the covering-glass a small distance from 

 it; and the object to be examined being placed on the slide with a drop 

 of water, the covering-glass is gently pressed down till it touches the 

 ring. Still thicker objects may be viewed in the various forms of t cells ' 

 hereafter to be described ( 171-3); and as, when the cells are filled 

 with fluid, their glass covers will adhere by capillary attraction, provided 

 the superfluous moisture that surrounds their edges be removed by blot- 

 ting-paper, they will remain in place when the Microscope is inclined. 

 An Annular Cell, that may be used either as a f live-box' or as a ' grow- 

 ing-slide,' has lately been devised by Mr. Weber (U. S.). It is a slip of 

 plate-glass of the usual size and ordinary thickness, out of which a cir- 

 -"i 'cell' of 3-4ths inch diameter is ground, in such a manner that 



Aquatic Box or Animalcule-Cage, as seen in 

 perspective at A, and in section at B and c. 



cular 



its bottom is convex instead of concave, its shallowest part being in the 

 centre, and the deepest round the margin. A small drop of the fluid 

 to be examined being placed upon the central convexity (the highest 



