28 CONSTRUCTION OF 



any intermediate angle; such, for example, as that shown in the 

 engraving. This moveable portion of the instrument consists of one 

 solid casting, D, E, F, G ; from F to G being a thick pierced plate, 

 carrying 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 H,. The piece 

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

 source of the required movement in the instrument, is obviously its 

 weakest part, and about which no doubt considerable 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 there is ge- 

 nerally 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. This is called the diaphragm. 

 Besides 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 generally understood ; and if not, will be rendered 

 more intelligible by a few minutes' examination of a microscope than 

 by the most lengthened description. One other point remains to be 

 noticed. The movement produced by the milled head K is not suffi- 

 ciently delicate to adjust the focus of very powerful lenses, nor indeed 

 is any rack movement ; only the finest screws are adapted for this 

 purpose, and even these are improved by means for reducing the 

 rapidity of the screw's movements. 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 downwards by a lever crossing the body, and acted on by an 

 extremely fine screw, whose milled head is seen at N, and the fineness 

 of which is tripled by means of the lever, through which it acts upon 



