1-18 MANAGEMENT OF THE MICEOSCOPE. 



of less than half- an -inch focus, it will be generally found con- 

 venient to employ the fine adjustment or 'slow motion ;' and as 

 recourse will frequently be had to its assistance for other purposes 

 also, it is very important that it should be well constructed and in 

 good working order. The points to be particularly looked to in 

 testing it, are for the most part the same with those already noticed 

 in relation to the coarse movement. It should work smoothly and 

 equably, producing that graduated alteration of the distance of 

 the Object-glass from the object which it is its special duty to effect, 

 without any jerking or irregularity. It should be so sensitive, that 

 any movement of the milled-head should at once make its action 

 apparent by an alteration in the distinctness of the image, when 

 high powers are employed, without any 'loss of time.' * And its 

 action should not give rise to any twisting or displacing move- 

 ment of the image, which ought not to be in the least degree 

 disturbed by any number of rotations of the milled-head, still less 

 by a rotation through only a few degrees. One great use of the 

 Fine adjustment consists in bringing into view different strata 

 of the object, and this in such a gradual manner that their con- 

 nection with one another shall be made appai-ent. "Whether an 

 Opaque or a Transparent object be under examination, only that 

 part which is exactly in focus can be perfectly discerned under any 

 power ; and when high powers of large angular aperture are 

 employed, this is the only part that can be seen at all. A minute 

 alteration of the focus often causes so different a set of appearances 

 to be presented, that, if this alteration be made abruptly, their 

 relation to the preceding can scarcely be even guessed at ; and the 

 gradual transition from the one to the other, which the Fine 

 adjustment alone affords, is therefore necessary to the correct in- 

 terpretation of either. To take a very simple case : — The trans- 

 parent body of a certain animal being transversed by vessels lying 

 in different planes, one set of these vessels is brought into view by 

 one adjustment, another set by 'focussing' to a different plane ; 

 and the connection of the two sets of vessels, which may be the 

 point of most importance in the whole anatomy of the animal, 

 may be entirely overlooked for want of a Fine adjustment, the 

 graduated action of which shall enable one to be traced con- 

 tinuously into the other. "What is true even of low and medium 

 powers is of course true to a still greater degree of high powers ; 

 for although the ' quick motion ' may enable the observer to bring 

 any stratum of the object into accurate focus, it is impossible 

 lor him by its means to secure that transitional ' focussing ' 

 which is often much more instructive than an exact adjustment 



* It will sometimes happen that the ' slow motion ' will seem not 

 to act, merely because it has been so habitually worked in one direction 

 rather than the other, that its screw has been turned too far. In that 

 case, nothing more is required for its restoration to good working order 

 than turning the screw in the other direction, until it shall have reached 

 about the middle of its range of action. 



