42 THE MICKOSOOPE. 



vertical position, or any intermediate angle, such, for instance, 

 as that shown in the engraving. 



This movable portion of the instrument consists of one solid 

 casting D E F G; from F to G being a thick pierced plate car- 

 rying the stage and its appendages. The compound body H is 

 attached to the bar D E, and moves up and down upon it by a 

 rack and pinion worked by either of the milled heads K. The 

 piece D E F G is attached to the pillar by the joint C, which 

 being the source of the required movement in the instrument, 

 is obviously its weakest part, and about which no doubt consid- 

 erable vibration takes place. But inasmuch as the piece 

 D E F G of necessity transmits such vibrations equally to the 

 body of the microscope and to the objects on the stage, they 

 hold always the same relative position, and no visible vibration 

 is caused, how much soever may really exist. To the under 

 side of the stage is attached a circular stem L, on which slides 

 the mirror M, plane on one side and concave on the other, to 

 reflect the light through the aperture in the stage. Beneath 

 the stage is a circular revolving plate containing three apertures 

 of various sizes, to limit the angle of the pencil of light which 

 shall be allowed to fall on the object under examination. Be- 

 sides these conveniences the stage has a double movement pro- 

 duced by two racks at right angles to each other, and worked by 

 milled heads beneath. It has also the usual appendages of 

 forceps to hold minute objects, and a lens to condense the 

 light upon them, all of which are well understood, and if not, 

 will be rendered more intelligible by a few minutes' examina- 

 tion of a microscope than by the most lengthened description. 

 One other point remains to be noticed. The movement pro- 

 duced by the milled head K is not sufficiently delicate to adjust 

 the focus of very powerful lenses, nor indeed is any rack move- 

 ment. Only the finest screws are adapted to this purpose; 

 and even these are improved by means for reducing the rapid- 

 ity of the screw's movement. For this purpose the lower end 

 of the compound body H, which carries the object-glass, 

 consists of a piece of smaller tube sliding in parallel guides 

 in the main body, and kept constantly pressed upwards by 

 a spiral spring but it can be drawn downward by a lever cross- 

 ing the body, and acted on by an extremely fine screw 



