350 HOW TO SEE WITH THE MICROSCOPE. 



concentric with the optical axis of the instrument by the lever 

 (Z),and the amount of such elevation or depression is registered 

 on a scale attached to the limb. This bar can be carried round 

 and above the stage, and be thus used for opaque illumination. 



The lower triangle bar (V) carries the mirror (H), or a right- 

 angle prism, when the illumination is required to be concentric 

 with the optical axis of the instrument, and independent of the 

 movements of other illuminating aparatus. 



The mirror- box contains two mirrors, one flat and the other 

 concave ; it swings in a rotating semi-circle attached to a 

 lengthening bar, which enables it to be turned from one side to 

 the other, and revolves on a circular ti tting for giving greater 

 lacilities in regulating the direction of the beam of light 

 reflected, the whole sliding upon either of the triangle bars, pre- 

 viously referred to, and made to reverse in the socket (a) so as 

 to bring the centre of the mirror concentric with the axis of the 

 microscope in either case. 



As the mirror alone is insufficient for many kinds of illumin- 

 ation, some provision h;is to be made for holding various pieces 

 of apparatus between ther object and the mirror. For this pur- 

 pose a supplementary body, or sub-stage, is mounted perfectly 

 true with the body, and is moved up and down in its fitting by 

 rack and pinion connected with the milled heads (W). This 

 sub-stage, to which reference has already been made, is now re- 

 garded as one of the most important parts of the achromatic 

 microscope; in it all the varied appliances for modifying the char- 

 acter and direction of the light are fitted. But a few years since 

 it was considered sufficient for this part of the stand to be con- 

 structed so as to move up and down perfectly coincident with 

 the optical axis of the instrument, and for that purpose it was 

 racked in a groove planed out on the same limb as that on the 

 upper end of which the optical portions were carried. But latelv 

 microscopists have shown the desirability of affording every 

 facility for lateral angular adjustments ; and this has led to the 

 sub-stage being attached to an arc (b) working in tbe circular 

 plate (Y), and moved by a rack and pinion (X), whilst the 

 amount of such angular movement is recorded on the upper sur- 

 face of the plate (Y). Having once fixed the angular direction 



