202 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



i- established liy means of a block of metal which fits upon the 

 pinion shaft and is pressed or released by means of the two screws 

 provided ti;r the purpose. This is shown in section in fig. 1(53, 

 where tlie pinion is P, the anti-friction block X, and one of the 

 adjusting screws M. The perspective view of the coarse adjustment 

 showing the adjusting screws is given in fig. 1(U. 



The stage can be completely rotated and has mechanical move- 

 ments on the Turrell principle, both milled heads being on one axis. 

 The snb-slai^e has a fine adjustment, and the plane mirror is eare- 

 fullv worked bv hand, while exceptional rigidity for the whole stand 

 is obtained bv a special system of construction, and the tripod, which 

 is shod with cork, has a spread often inches. 



A high -class stand of distinguished merit is made by the firm of 

 Uaker of llolhorn. It is illustrated in fig. 165, is made with great 



FIG. 164. Ci'in|.Irtr view of Watson's coarse adjustment (1895). 



care and isan instrumenl of precision. It is mounted on a solid tripod 

 with slotted toes so that it can be firmly clamped to the baseboard 

 of a photo-micrographic apparatus. The body is mounted on a mas- 

 sive limb in our piece throughout, and on to this the stage and 

 sub-stage are mounted; in this way the chance of derangement of 

 the optic axis is reduced to a minimum. The body has diagonal 

 rack-and-pinion coarse adjustment actuated by very large milled 

 heads, making a slow movement easy. The line adjustment carries 

 the body tube only each revolution of the graduated milled head, 

 bein- e<|ual to the ._,,',,,"' of an inch ; the Campbell differential screw 

 being employed, and the milled head being placed at the lower end 

 of the body. The body ran l,e extended to .".(M) mm. and closed to 

 The mechanical stage is worked on the Turrell method 

 stationary milled heads working on a common centre commanding 



obli<|iieas well as red a ngula r mo\ ement s ; the rectangular movements 

 \ided silver plates for recording positions: and complete rota- 

 ion can be secured, either by hand or by rack and pinion, and ran 



be a I any point clamped. 



sub stage lias rectangular mechanical movements controlled 



