1896.] MICROSCOPICAL JOURNAL. 147 



and aft. The side movement is obtained as follows : — A suit- 

 able length of pinion wire has a piece of brass tube pusiied on 

 tight midway. The ends of this are turned away so that it just 

 fits between the inner sides of the two parallel racks. The ex- 

 terior of the tube is turned and polished quitrs true. Now mark 

 off the pinion wire to the outside of the racks, and turn all the 

 teeth away right to the ends. Make these blanks quite parallel 

 and polish them nicely. All this is best done with a clock- 

 maker's hollow centre turn, worked with a bow, in a manner 

 familiar to experts : these polished ends pass through suitable 

 angle bearings, screwed up from beneath the stage, as shown in 

 side view Fig. 3. What we now require is to keep the top plate 

 down in contact with the bottom one,- by a fine elastic pressure. 

 The pinion is set in its bearings, and properly geared with the 

 rack at a sufficient height to allow a thin hammered brass spring 

 to be inserted beneath ; — this is bowed up so as to b-ar up in 

 the middle of the brass pinion-sheath ; the ends consequently 

 press down on the two sides of the upper plate. To keep this 

 spring- in place two blocks are fixed to the ends, rising a Jittle 

 above the centre of the pinion-sheath, and cut out so as to em- 

 brace it as shown in plan and side view, Fig. 4, in which the 

 under curved black line shows the spring. No oil must be ap- 

 plied to this stage, and the lower surface of the top jjhite must 

 be quite flat with the under one. Turning with a good slide- 

 rest will eff.^ct this, and finish by stoning over. The surfaces 

 may be smeared with blacklead. 



As to tiie two outside milled heads, they are simply driven 

 on to the ends of the pinions, which are very slightly tapered 

 and held up by fine screws tapped therein. 



At first sight it might be inferred that the sliding transverse 

 movement would not harmonise with the rack-and-pinion one ; 

 but the first trial will prove that this is not the case. The 

 erratic movements of an aquatic animalcule can be followed up 

 at once with perfect ease ; — in fact it acts as easily (at least, in 

 my hands) as any other rectilinear mechanical stage. The 

 movement is too simple to be misunderstood; but I trust that 

 I have been sufficiently explicit. — F. H. Wenham, in English 

 Mechanic. 



