138 THE MICROSCOPE AND ITS REVELATIONS. 



unless a spring ' safety-tube ' be provided, into which the Objectives are 

 screwed. It is here, perhaps, well to notice, for the guidance of the 

 young Microscopist, that the actual distance between the Objective and 

 the object, when a distinct image is formed, is always considerably less 

 than the nominal focal length of the objective. One more precaution it 

 may be well to specify; namely, that either in changing one object for 

 another, or in substituting one Objective for another, save when powers 

 of such focal length are employed as to remove all likelihood of injury, 

 the Body should have its distance from the Stage increased by the 

 * coarse adjustment.' This precaution is absolutely necessary when 

 Objectives of short focus are in use, to avoid injury either to the lenses 

 or to the object; and when it is habitually practised with regard to these, 

 It becomes so much like an i acquired instinct/ as to be almost invariably 

 practised in other cases. 



138. In obtaining an exact Focal Adjustment with Objectives of less 

 than half-an-inch focus, it will be generally found convenient to employ 

 the fine adjustment or 'slow motion;' and as recourse will frequently be 

 liad 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 Objective 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 movement 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 this 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 apparent. 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, the relation of each to its predecessors can scarcely be 

 even guessed at; and the gradual transition from the one to the other, 

 which the ' slow motion ' alone affords, is therefore necessary 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 over- 

 looked for want of a 'fine adjustment,' whose graduated action shall 

 enable one to be traced continuously into the other. What is true even 



1 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. 



