164 MANAGEMENT OF THE MICROSCOPE. 



time." 1 And its action should not give rise to any twisting or 

 displacing movement of the image, which ought not to be in the 

 least decree 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 connection with one another shall be made apparent. 

 "Whether an opaque or a transparent object be under examina- 

 tion, only that part can be perfectly discerned under any power, 

 which is exactly in focus ; and when high powers of large aper- 

 ture are employed, this is the only part that can be seen at all. 

 A minute alteration of the focus often causes so entirely 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 ne- 

 cessary to the correct interpretation of either. To take a very 

 simple case : The transparent body of a certain animal being 

 traversed 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 continuously 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 " coarse 

 movement" may enable the observer to bring any stratum of the 

 object into accurate focus, it is impossible for him by its means 

 to secure that transitional "focussing," which is often so much 

 more instructive than an exact adjustment at any one point. A 

 clearer idea of the nature of a doubtful structure is, in fact, often 

 derived from what is caught sight of in the act of changing the 

 focus, than by the most attentive study and comparison of the 

 different views obtained by any number of separate " focussings." 

 The experienced Microscopist, therefore, when examining an 

 object of almost any description, constantly keeps his finger 

 upon the milled head of the "fine movement," and watches the 

 effect produced by its revolution upon every feature which he 

 distinguishes ; never leaving off, until he be satisfied that he has 

 scrutinized not only the entire surface, but the entire thickness of 

 the object. It will often happen, that, where different structural 

 features present themselves on different planes, it will be difficult 

 or even impossible to determine which of them is the nearer and 



*It will sometimes happen that the "fine movement" 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. 



