THE MICROSCOPE. 
object with perfect distinctness; this will take place provided the 
mutual distances between the eye-piece, the object-piece, and the 
object are suitably adjusted; and this adjustment may be accom¬ 
plished by moving any one of these three towards or from the 
other two, while these last remain fixed: thus, for example, if the 
object and the object-piece remain unmoved, the instrument may 
be brought into focus by moving the eye-piece to or from the 
object-piece. The rack and pinion, already described, which 
moves the tube in which the eye-piece is inserted, can accomplish 
this. This provision, however, is not made in all microscopes. 
If the eye-piece and the object be fixed, the instrument may be 
brought into focus by moving the object-piece to or from the 
object. To effect this, it would be necessary that the object-piece 
should be inserted in a tube, moved by a rack and pinion, like 
that of the eye-piece. 
In fine, if the object-piece and eye-piece be both fixed, the 
instrument may be brought into focus by moving the object, or 
whatever supports it, to or from the object-glass. 
All these methods are resorted to in the different forms in 
which microscopes are mounted by different makers. 
25. Since nearly all material substances, when reduced to an 
extreme degree of tenuity, are more or less translucent, and since 
almost all microscopic objects have that degree of tenuity by 
reason of their minuteness, it happens that nearly all of them are 
more or less translucent; and where in exceptional cases a 
certain degree of opacity is found, it is removed without inter¬ 
fering with its structure, by saturating the object with certain 
liquids, which increase its translucency, just as oil renders paper 
semi-transparent. The liquid which has been found most useful 
for this purpose, is one called Canada balsam. When the object 
is saturated with this liquid, it is laid upon a slip of glass, about 
two inches long and half an inch wide, and is covered with a 
small piece of very thin glass, made expressly for this purpose, 
the thickness in some cases not exceeding the 100th of an inch. 
It is usual to envelope the oblong slip of glass, in the middle of 
which the object is thus mounted with paper gummed round it, a 
small circular hole being left uncovered on both sides of the 
glass, in the centre of which the object lies. 
The slips of glass thus prepared, with the objects mounted 
upon them, are called slides or sliders ;' and the objects thus 
mounted are so placed, that the axis of the object-piece shall be 
directed upon that part of them which is submitted to observa¬ 
tion, provisions being made to shift the position of the slider, so 
as to bring all parts of the object successively under observation. 
Further provisions are also made to throw a light upon the 
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