MICROSCOPIC INSTRUMENTS. 361 



A, compound body ; B, milled-head for fine adjustment; 

 C, three achromatic object-glasses; D, moveable stage ; E, 

 milled-head screws for moving stage ; F, mirror for illu- 

 minating the object ; G, milled-head for moving mirror ; 

 H, frame for holding polarizing apparatus ; I, lever for 

 moving frame of polarizing apparatus ; K, condenser for 

 illuminating opaque objects; L, test-object; M. microm- 

 eter ; N, achromatic object-glass of low power ; O, three 

 achromatic lenses medium power ; P, three achromatic 

 lenses high power; R, dissecting-point ; S, dissecting- 

 knife ; T, forceps. 



The mirror, F, consists of a frame of brass, in which 

 are set two silvered glasses, one concave and the other 

 plane ; the former reflects the light in converging, and 

 the latter in parallel rays. For proper adjustment the 

 frame and glasses are made to turn on joints, so that the 

 rays reflected may be made to fall upon any given object 

 on the stage. The tube may be turned around on the 

 foot, and the stage on the tube, so that not only can the 

 light falling on the mirror be put in any situation, but the 

 stage, and with it the object, can be revolved, and rays, 

 however oblique, may be thrown on all sides of any 

 object. Beneath the stage-plate, and within the tube, is 

 a frame, H, with a circular opening in the centre for re- 

 ceiving various appendages to the instrument, as the dia- 

 phragm, and polarizing apparatus ; this frame is jointed 

 so that it may be raised and depressed by the lever, I. 

 To the stage-plate is fixed the support for the compound 

 body, A, which is a brass tube about seven inches long, 

 and containing the object-glass at the lower end, and the 

 eye-piece at the upper end. To the stage-plate are fixed 

 two brass springs, about two inches long, moveable from 

 the stage by pins fitting sockets in the stage-plate, by 

 which any slider placed beneath, and the springs pressed 

 thereon, may be confined ; and when required to turn the 

 microscope in a horizontal position, they are not dis- 

 placed. There are three eye-pieces belonging to these 

 instruments, of different powers; each one consists of two 

 plano-convex lenses placed at a distance from each other, 

 equal to half the sum of their focal lengths ; the plane 

 surfaces of the lenses are towards the eye, and the nearest 

 the eye is called the eye-glass, while that more distant is 

 called the field-glass. A stop or diaphragm is placed 



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