1896] MICROSCOPICAL JOURNAL. 147 



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

 able length of [)inion wire has a |)iece of brass tube [)ushe(l on 

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

 tits between the inner sides of the two {)arallel racks. The ex- 

 terior of the tube is turned and polished quit-! true. Now mark 

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

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

 and jiolish tliem 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 suital)le 

 angle bearings, screwed up truin 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 tine elastic pressure. 

 The pinion is s>it in its bearings, and properly geared with the 

 rack at a sufficient height to allow a thin hammered l>rass 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 little 

 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 tliis stage, and the lower surface of the top plate must 

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

 rest will effjct this, and finish by stoning over. The surfaces 

 may be smeared with blacklead. 



As to the tw^o outside milled heads, they are simi)ly driven 

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

 and held up l)y fine screw^s tapped therein. 



At first sight it might be inferred that the slitliiig 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. 



