ROSS'S LARGE COMPOUND MICROSCOPE. 109 



(an instrument contrived to indicate otherwise insensible vibra- 

 tions), and either strengthened or reduced as might be found 

 necessary, so as to obtain an equality ^ of vibration between the 

 stage and the optical part, which will prevent any percepti- 

 ble tremor in the image. The " coarse" adjustment is made by 

 the large milled head situated just behind the summit of the up- 

 rights, which turns a pinion working into a rack cut on the back 

 of a very strong flattened stem, that carries the transverse arm at 

 its summit ; a second milled head (which is here concealed by 

 the stage fittings) is attached to the other end of the axis of the 

 pinion (as in Fig. 18), so as to be worked with the left hand. The 

 " fine" adjustment is effected by the milled head on the transverse 

 arm just behind the base of the "body;" this acts upon the 

 " nose" or tube projecting below the arm, wherein the objectives 

 are screwed. The other milled head seen at the summit of the 

 stem, serves to secure the transverse arm to this, and may be 

 tightened or slackened at pleasure, so as to regulate the travers- 

 ing movement of the arm ; this movement is only allowed to 

 take place in one direction, namely, towards the right side, being 

 checked in the opposite by a "stop," which secures the coinci- 

 dence of the axis of the body with the centre of the stage and 

 with the axis of the illuminating apparatus beneath it. It is in 

 the movements of the stage, that the greatest contrivance is 

 shown ; these are three, namely, a traversing movement from 

 side to side, a traversing movement from before backwards, and 

 a rotatory movement. The traversing movements, which allow 

 the platform carrying the object to be shifted about an inch in 

 each direction, are effected by the two milled heads situated at 

 the right of the stage ; and these are placed side by side, in such 

 a position that one may be conveniently acted on by the fore- 

 finger, and the other by the middle finger, the thumb being 

 readily passed from one to the other. The traversing portion 

 of the stage carries the platform whereon the object is laid, which 

 has a ledge at the back for it to rest against ; and this platform 

 has a sliding movement of its own, from before backwards, by 

 which the object is first brought near to the axis of the micro- 

 scope, its perfect adjustment being then obtained by the traversing 

 movement. To this platform, and to the traversing slides which 

 carry it, a rotatory movement is imparted by a milled head, 

 placed underneath the stage on the left hand side ; for this milled 

 head turns a pinion which works against the circular rack (seen 

 in the figure) whereby the whole apparatus above is carried round 

 about a third of a revolution, without in the least disturbing the 

 place of the object, or removing it from the field of the micro- 

 scope. This rotatory movement is useful for two purposes ; first, 

 in the examination of very delicate objects by oblique lights, in 

 order that, without disturbing the illuminating apparatus, the 

 effect of the light and shadow may be seen in every direction, 

 whereby important additional information is often gained ; and, 



