172 MICROSCOPIC ILLUSTRATIONS. 



the upright triangular bar, b, and is kept in its place by 

 the gun-metal garter-piece, shewn at 'a, and has a saddle- 

 piece well screwed on, covering it over, to prevent the 

 loosening the pinion, by the action of the milled lever. 

 The endless screw may be detached from the wheel, t, 

 hy slackening the pinching screw, b, which serves to keep 

 the pinion in gear, the said endless screw being affixed 

 to a separate piece, which turns round on the project- 

 ing and inferior part of the socket, by means of a pivot : 

 the pinching screw, 'b, operating in an arc, formed to 

 receive it. This wheel and endless screw are absolutely 

 necessary to enable us to command the motions of the 

 arm, when the axis of the body of the instrument is not 

 perpendicular, or nearly so. 



The body, a, is constructed on the smallest scale on 

 which it can well be made without impairing its optical 

 properties. The only true way of preventing the genera- 

 tion of false light is to allow so much space in the body that 

 the rays proceeding from the object-glass shall not impinge 

 on any part of the tube ; for no sensible light is produced 

 until they flash against something. Into the superior part 

 of the eye-tube, 'c, are screwed the various inverting eye- 

 glasses, and into its inferior part, 'd, the erecting glasses, 

 A. Upon the neck of the object end, 'e, are screwed the 

 various object-glasses, oneof which is represented attached 

 in the plate. Upon this neck also slides the illuminator, '/, 

 for opaque objects ; it has a split socket and clamping 

 screw, '(/, by which it is adjusted, and is composed of a 

 plain convex lens, having an inch and a half of aperture 



