HiSLOP, on a New Secondary Stage. 
195 
adjustable in two directions for centering-, and vertically to 
the stage for focusing. 
Eio-. I. 
Fig. 1 shows the contrivance attached to the instrument, 
and fig. 2 when separated from it. 
It consists of a base plate. A, which slides beneath the 
stage. A limb is attached at right angles to this plate, 
which limb is ploughed out for a slide actuated by a rack 
and pinion, the milled head of which is seen at B. On this 
slide^ and at right angles to " its plane of motion,^^ are affixed 
a pair of cheeks, between which a smaller slide, carrying the 
achromatic condenser, Nicol's prism, spotted lens, &c.j is 
inserted. The base plate has an adjusting screw at C, which 
centres in one direction by bearing against the stage; and at 
VOL. VI. / ^ 
