MANAGEMENT OF THE MICROSCOPE. 



139 



of low and medium powers, is of course true to a still greater degree as 

 to high powers; for although the 'quick motion' 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 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 differ- 

 ent views obtained by any number of separate ' f ocussings. ' The experi- 

 enced Microscopist, therefore, whilst examining an object of almost any 

 description, constantly keeps his finger on the milled-head of the 'slow 

 motion/ 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 thick- 

 ness 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 with the Monocular microscope which of them is 

 the nearer and which the more remote, unless it be ascertained by the use 

 of the 'slow motion/ when they are successively brought into focus, 

 whether the Objective has been moved toivards or aivay from the object. 1 

 Even this, however, will not always succeed in certain of the most diffi- 

 cult cases, in which the difference of level is so slight as to be almost 

 inappreciable; as, for instance, in the case of the markings on the silice- 

 ous valves of the Diatoms (Fig. 166). 



139. When Objectives of short focus and of wide angular aperture are 

 in use, something more is necessary (save in the case of ' homogeneous- 

 immersion' lenses, 20), than exact focal adjustment; this being the 

 adjustment of the Objective itself, 



which is required to neutralize the no. in.. 



disturbing effect of the glass cover 

 upon the course of the rays pro- 

 ceeding from the object ( 17), 

 unless (as in the Objectives now 

 commonly made for Students' 

 Microscopes) they are construct- 

 ed for working only with cover- 

 glasses of a certain standard thick- 

 ness. For such adjustment, it 

 will be recollected, a power of al- 

 tering the distance between the 

 front pair and the remainder of 

 the combination is required; and 

 this power is obtained in the fol- 

 lowing manner: The front pair 

 of lenses is fixed into a tube (Fig. 

 Ill, A), which slides over an 

 interior tube (B) by which the 



other two pairs are held; and it is drawn up or down by means of a col- 

 lar (c), which works in a furrow cut in the inner tube, and upon a screw- 

 thread cut in the outer, so that its revolution in the plane to which it 

 is fixed by the one tube gives a vertical movement to the other. In one 



Covered- 



Section of Adjusting Object-Glass. 



1 It is in objects of this kind that the great advantage of the Stereoscopic 

 ^Binocular arrangement makes itself most felt ( 30-40). 



