ADJUSTMENT OF FOCUS, AND OF OBJECT-GLASS. 149 



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, 

 whilst examining an object of almost any description, constantly 

 keeps his finger upon 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 (§ 86), unless it be ascertained by the use of the ' slow 

 motion,' when they are successively brought into focus, whether 

 the Object-glass has been moved towards or away from the object/' 

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

 difficult 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 siliceous lorico? of the Diatomacere (§ 127). 



113. Whett Objectives of short fccus and of wide Angular Aper- 

 ture are being employed, something more is necessary than exact 

 focal adjustment ; this being the Adjustment of the Object-glass 

 itself, which is required to neutralize the disturbing effect of the 

 glass cover upon the course of the rays proceeding from the object 

 (§ 15). For this adjustment, it will be recollected, a power of 

 altering the distance between the front pair and the remainder of 

 the combination is required ; and this power is obtained in the 

 following manner: — The front pair of lenses is fixed into a tube 

 (Fig. 86, 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 collar (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 move- 

 ment to the other. In one part of the outer tube an oblong slit is 

 made, as seen at d, into which projects a small tongue screwed on 

 the inner tube ; at the side of the former two horizontal lines are 

 engraved, one pointing to the word 'uncovered,' the other to the 

 word ' covered ;' whilst the latter is crossed by a horizontal mark, 

 which is brought to coincide with either of the two lines by the 

 rotation of the screw-collar, whereby the outer tube is moved up or 

 down. When the mark has been made to point to the line ' un- 

 covered,' it indicates that the distance of the lenses of the object- 

 glass is such as to make it suitable for viewing an object without 

 any interference from thin glass ; when, on the other hand, the 



* It is in objects of this kind that the great advantage of the Stereo 

 scopic Binocular arrangement makes itself most felt (§ 31). 



