OF THE MICROSCOPE. 53 



This principle of arranging the various parts to revolve round 

 the object as a center, as it were, was carried out in the most 

 complete manner by Mr. "Wenham in his concentric model, a 

 simplified form of which, as made by the Bausch & Lomb Op- 

 tical Co., is shown in Plate VII. 



;t The Biological Stand of Mr. Bullock is shown in Plate III. 

 It will be seen at once that this stand is substantial and firm 

 in all its parts. The coarse movement is by rack and pinion, 

 and the body is supported, as in the Jackson model, along 

 nearly its entire length. The fine movement acts upon the 

 entire body, the coarse movement included, so that the distance 

 between the eye-piece and the objective is not affected by it. 

 The stage is thin, but substantial, and is so cut away at the 

 well-hole that light of great obliquity can be used. The stage- 

 plate rotates in a ring, and may be clamped when necessary by 

 means of the small screw seen in front. By loosening this 

 screw the plate may be entirely removed, and then, when the 

 body stands upright, a simple plate of glass may be used for a 

 stage, and thus all danger of injury from acids is avoided. The 

 ring in which the stage-plate rotates is held in place by two 

 capstan-headed screws, and when these are slacked it is easy 

 to adjust the stage so that it shall be concentric with the optic 

 axis. The sub-stage is arranged for adjustment in the same 

 way. It is adapted to carry the standard size accessories, and 

 is furnished with an adapter which has the Society screw, so 

 that ordinary objectives may be used as condensers. Mr. 

 Bullock also furnishes a diaphragm of peculiar form (shown in 

 the engraving), which we have found very efficient, both with 

 direct and oblique light. The special feature of models of this 

 class is, however, shown in the method of hanging the mirror 

 and sub-stage. These are attached to separate bars, which 

 rotate on an axis, the plane of which is a little above the level 

 of the stage. A stop is also provided, whereby they may both 

 be placed precisely in the line of the optic axis. Both mirror 

 and sub-stage may be moved towards the stage and from it, and 

 the movements of both are entirely independent. The conse- 

 quence is that the various forms of illuminating apparatus have 

 a range of adjustment that cannot be obtained with the older 

 models. 



