80 FIRST-CLASS MICROSCOPES. 



the Achromatic Condenser. The Mirror has a doubly-extending 

 arm ; and can be so placed as to reflect light upon the object from 

 outside the large brass ring that supports the stage and sub-stage. 

 Light of the greatest obliquity, however, may be more conveniently 

 obtained by an Amici's prism (Fig. 62) placed above the supporting 

 ring. — Notwithstanding the weight of all this apparatus, the in- 

 strument is so well balanced on its horizontal axis, that it remains 

 perfectly steady without clamping, in whatever position it may be 

 placed. And in regard to the apparent complexity of its arrange- 

 ments, the remarks already made upon Mr. Ross's instrument are 

 equally applicable to the one now described. 



55. Smith and Beck's First-class Microscope. — For the general 

 plan of this Instrument, the Author has already expressed his 

 preference (§ 39 ) : the support of the Body, along a large propor- 

 tion of its length, upon the substantial Limb to which the Stage is 

 securely attached, giving it a decided advantage in steadiness over 

 any form of instrument (not exceeding it in massiveness) in which 

 the Body is attached at its lower extremity only to an Arm be- 

 tween Avhich and the Stage there is no fixed connection ; whilst the 

 Rack-and-pinion movement giving the ' coarse ' adjustment can be 

 made to work more easily on this construction, than where it is 

 requisite that the stem moved by it should be fitted as tightly 

 as possible. On the other hand, it must be admitted that the 

 ' fine ' adjustment can be more effectually made by the longer lever- 

 age provided in the ' Ross ' model, than by the attachment of the 

 screw to the lower end of the Body as in the instrument before 

 us. The Stage, which is furnished with the usual traversing 

 movements, is made (by an arrangement first devised by Messrs. 

 Smith and Beck, ar 1 since adopted by other makers) so thin as to 

 allow of extremely oblique illumination ; but although the plat- 

 form which carries the object can be made to rotate upon the 

 traversing apparatus, yet the object is liable to be thrown out of 

 centre by this rotation, so as to require continual re-adjustment 

 by the traversing motion ; and the stage as a whole is without the 

 rotatory movement given to it in the Microscope of Mr. Ross, and 

 in the Large Microscope of Messrs. Powell and Lealand. This want 

 of the power of rotation in the optic axis of the instrument 

 cannot but be regarded (for the reasons already stated, §§ 48, 52) 

 as a decided deficiency in an instrument that is otherwise most 

 complete and effective. 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, which answers the same purpose as the ' secondary 

 stage' of Mr. Ross's Microscope. Being made to work in a groove 

 which is in perfect correspondence with that wherein the principal 

 ' body' works (this correspondence being secured by the action of 



