88 PRACTICAL TREATISE ON THE MICROSCOPE. 



at its back, the movement of the body up and down the arm is 

 effected by the pinion connected with the milled heads, Gr G, 

 which form the coarse adjustment. There is a draw-tube at 

 the upper end of the body, into which the eye-pieces and 

 erecting-glass fit, and to the lower end there is added a short 

 tube, to carry the object-glasses ; this is moved up and down 

 slowly by the nut, K, acting on the end of a lever, and so 

 forms the fine adjustment. The stage adapted to this instru- 

 ment may be one of two forms, either one whose movements 

 are effected by a lever, or else so constructed that the up and 

 down motion is produced by a rack and pinion, and that from 

 side to side by a screw, whose axis is carried across to the 

 opposite side of the stage, and there can be turned by the left 

 hand. The lever stage is represented as attached to the 

 instrument ; this is constructed after the plan of that of Mr. 

 Alfred White, and described by him in VoL I. of the Trans- 

 actions of the Microscopical Society. It consists of three plates 

 of brass, the lower one of which is fixed, and the other two 

 provided with certain dove-tailed guides and slides, so that 

 the upper one may be moved by a lever, either independently 

 of the middle one, or else be carried along with it. The lever 

 is seen at O ; it is about five inches long, and is loaded with 

 metal at its upper part, so as to balance the weight of the 

 stage-plate, and at its lower end is provided with a baU work- 

 ing in a socket connected with the upper plate; about an 

 inch higher up is another ball working in a socket, P, in a 

 small arm connected with the support of the compound body, 

 C C. The dove-tail guides of the middle stage-plate are 

 arranged horizontally, whilst those of the upper plate are 

 placed vertically; when, therefore, the lever, o, is moved 

 either to or from the support of the compound body, both 

 stage-plates will move horizontally in the opposite direction 

 but when the lever is moved in a hue parallel with the side of 

 the same support, then only the upper one is moved ; and as 

 the end of the lever to which the hand is applied moves in all 

 cases in an opposite direction to that of the ball, a, and as the 

 compound microscope always inverts the image of the object 

 under examination, the object wUl appear to move in the 



