POWELL AND LEALAND's FIRST-CLASS MICROSCOPES. TO 



instead of beneath the traversing apparatus, there is no security 

 that the centre of rotation shall coincide with the optic axis 

 of the instrument ; so that, unless this adjustment have been 

 previously made, the object will be thrown completely out of 

 the field of view when the platform is made to revolve. The Con- 

 denser for transparent objects, the Polarizing apparatus, &c, are 

 here fitted to the under 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 xipon two centres, is pivoted 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 attached 

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

 that suitable degree of stiffness which shall prevent it from being 

 disturbed by a passing touch. No instrument can be better adapted 

 than this to all the ordinary wants of the Microscopist ; there are 

 very few purposes which it cannot be made to answer : and there 

 are many who will consider that its deficiency as to these is counter- 

 balanced (to say the least) by its comparative simplicity and porta- 

 bility, 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, with rotatory movements of the Stage and Sub-stage, 

 Messrs. P. and L. have brought out a new pattern (Plate vi.), 

 which, while it resembles the preceding in its general plan of con- 

 struction, though much more massive, differs from it entirely in 

 the construction of the stage and sub-stage, both of which rest on 

 the foundation of a large solid brass ring, firmly attached to the 

 stem of the instrument. The upper side of this ring bears a sort 

 of carriage that supports the Stage ; and to this carriage a rotatory 

 movemeut is given by a milled-head, the amount of the movement 

 (which may be carried through an entire revolution) being exactly 

 measured by the graduation of a circle of gun -metal, which is borne 

 on the upper surface of the ring. The rotatory action of the Stage 

 being thus effected beneath the traversing movement, the centering 

 of an object brought into the axis of the Microscope is not disturbed 

 by it ; and the workmanship is so accurate, that the stage may be 

 made to go through its whole revolution without throwing out of 

 the field an object viewed even with the 1-1 6th inch objective. The 

 Stage, which is furnished with the usual traversing movements, is 

 made thin enough to admit of the most oblique light being thrown 

 on the object ; and it is furnished with graduated scales, so that 

 the place of any particular object can be registered without the use 

 of a 'finder' (§ 76). The Sub-stage also is furnished with rota- 

 tory and rectangular, as well as with vertical movements ; and, 

 like that of Messrs. Smith and Beck, it is made in such a manner 

 as to admit of the simultaneous use of the Polarizing pi-ism and of 



