ROSS'S FIRST-CLASS MICROSCOPE. 77 



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

 or slackened at pleasure, so as to regulate the traversing 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 coincidence 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 tbree, 

 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 carry- 

 ing 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 Microscope, 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 two-thirds of a revolution, without 

 in the least disturbing the place of the object, or removing it 

 from the field of the Microscope. The graduation of the circular 

 rack, moreover, enables it to be used as a Goniometer (§ 69). In 

 the improved form of this instrument here represented, the whole 

 Stage -apparatus is made so thin, and the opening beneath so large, 

 as to permit the employment of light of extreme obliquity ; and to 

 enable the Mirror to afford this, it is mounted upon an extending 

 arm, the socket of which slides upon a cylindrical stem. Below 

 the stage, and in front of the stem that carries the mirror, is a 

 dovetail sliding- bar, which is moved up and down by the milled- 

 head shown at its side ; this sliding-bar carries what is termed by 

 Mr. Ross the 'Secondary Stage' (shown separately at b), which con- 

 sists of a cylindrical tube for the reception of the Achromatic Con- 

 denser, Polarizing prism, and other fittings ; it is here shown as 

 fitted with a Condenser specially devised by Mr. T. Ross for the 

 illumination of a large field under low magnifying powers. To this 

 Secondary Stage, also, a rotatory motion is communicated by the 

 turning of a milled-head ; and a traversing movement of limited 

 extent is likewise given to it by means of two screws, one on the 

 front and the other on the left-hand side of the frame which carries 

 it, in order that its axis may be brought into perfect coincidence with 



