ELEMENTS OF MICROSCOPY. 121 



turning ; this point therefore requires attention. The tube should 

 be of sufficient diameter, otherwise the field of view is seriously 

 limited by the consequent smallness of the eye-piece, and this is 

 the great disadvantage of the very small stands which are now 

 sold ; since a small eye-piece is, like a large one, most accurate at 

 or near its centre, and the area of flatness of field becomes 

 extremely small. 



Of adjustments — i.e.^ of means of focussing — there are usually 

 two, the coarse and the fine, or, as they are often called, the quick 

 and the slow motions. The coarse adjustment is, in the cheaper 

 stands, often effected by a sliding arrangement, the tube of the 

 microscope itself being moveable within a tube attached to the 

 body of the microscope, but, except in point of cheapness, this 

 plan has nothing to recommend it. On the contrary, many a 

 valuable slide has been ruined by a sudden descent of the object- 

 glass (which has also frequently suffered) to a point lower than 

 had been calculated for or expected. The small extra expense of 

 the rack and pinion is more than compensated by the facility and 

 security in working obtained by its use. Not unfrequently the 

 optical tube itself is made in two parts, the upper sliding within the 

 lower, and this is in some respects an advantage, since it offers a 

 considerable range of variation in magnification, without varying 

 the eye-piece or objective employed, though, unless the tubes be 

 of fair size, with a correspondingly large eye-piece, the advantage 

 is counterbalanced by a great want of correctness in the outer 

 portions of the field of view. 



The fine adjustment, or slow motion, by means of which 

 accurate adjustment of the focus is made, so as to secure the 

 utmost possible sharpness of image, is of very varied character, 

 and its arrangement depends somewhat upon the general construc- 

 tion of the stand. It would serve no useful purpose here to go 

 into mechanical details, but the Campbell fine adjustment, it may 

 be said, which is effected by means of a screw, which has two 

 threads upon different parts of the same rod, working in opposite 

 directions, has a great advantage in point of steadiness and 

 wearing powers over any except the very best of the lever fine 

 adjustments, if it be not indeed as good as any of them, and it is, 

 moreover, not expensive in proportion to its goodness. The 



Journal of Microscopy and Natural vScience. 



New Series. Vol. III. 1890. k 



