112 CONSTRUCTION OF THE MICROSCOPE. 



side of the principal stage itself, instead of to an independent 

 or secondary stage ; an arrangement which, though convenient 

 as regards compactness, admits of less variety of adjustment 

 than is afforded by the latter plan. The mirror, instead of being 

 swung loosely upon two centres, is pivofed to one end of a 

 quadrant of brass, of which the other end is pivoted to a strong 

 pin that projects from the sliding tube ; a spring being so at- 

 tached to each of these pivots, as to give to the movements of 

 the mirror that suitable degree of stifthess, which shall prevent 

 it from being disturbed by a passing touch. ISTo instrument can 

 be better adapted than this to all the ordinary wants of the Mi- 

 croscopist ; there are very few purposes which it cannot be made 

 to answer ; and there are many who will consider that its defi- 

 ciency as to these is counterbalanced (to say the least) by its 

 comparative simplicity and portability, as well as by its lower 

 cost. For the sake, however, of such as may desire the power 

 of obtaining a more oblique illumination, than is permitted by 

 the construction of the stage in the instrument just described, 

 Messrs. P. and L. have recently brought out a new pattern, in 

 which the thickness of the stage is greatly reduced, a sub-stage 

 is provided for the reception of the condenser and other fittings, 

 and the mirror is mounted on a doubly extending arm. 



39. Smith and Beck's Large Microscope. — The general plan of 

 this instrument (Fig. 29) nearly resembles that of the " dissecting 

 microscope" of the same makers, already noticed (§ 35), so far, 

 at least, as regards the mode of supporting the body, and of 

 effecting the focal adjustments; whilst in the construction of the 

 stage, and in the arrangement of the fittings beneath, it differs 

 from all the microscopes hitherto described. The stage is fur- 

 nished with the usual traversing movements ; but it is distin- 

 guished by its thinness; and this is of importance in certain 

 cases, as admitting of a more oblique illumination than could 

 otherwise be obtanied, and also as allowing the construction of 

 the achromatic condenser (§ 56) to be much simplified. The 

 platform for the object is fitted upon the traversing apparatus, in 

 the same mode as in the microscope last described, and possesses 

 the same kind of rotatory movement. Beneath the stage is a 

 continuation of the gun-metal "limb" which carries the body; 

 and this is ploughed out into a groove for the reception of a 

 sliding-bar, which carries what may be termed the " secondary 

 body," namely, a short tube (seen beneath the stage), capable of 

 being_ moved up and down by a milled head, and fitted for the 

 reception of the achromatic condenser, polarizing apparatus, &c. 

 This " secondary body" consequently answers the same purpose 

 as the " secondary stage" of Mr. Eoss's microscope, and its rela- 

 tions to the other parts of the instrument are essentially the 

 same ; but it differs in the following particulars :— first, that by 

 being made to work in a groove which is in perfect correspon- 

 dence with that wherein the principal "body" works (this corre- 



