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 thft 

 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 t@ throw a light upon the 

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