202 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE 



is established by means of a block of metal which fits upon the 

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

 provided for the purpose. This is shown in section in fig. 163, 

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

 adjusting screws M. The perspective view of the coarse adjustment 

 showing the adjusting screws is given in fig. 164. 



The stage can be completely rotated and has mechanical move- 

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

 The sub-stage has a fine adjustment, and the plane mirror is care- 

 fully worked by hand, \vhile exceptional rigidity for the whole stand 

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

 is shod with cork, has a spread of ten inches. 



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

 Baker of Holborn. It is illustrated in fig. 165, is made with great 



FIG. 164. Complete view of Watson's coarse adjustment (1895). 



care and is a.n instrument 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 one 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 fine adjustment carries 

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

 being equal to the ^^ ih of an inch ; the Campbell differential screw 

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

 of the body. The body can be extended to 300 mm. and closed to 

 150 mm. The mechanical stage is worked on the Turrell method 

 by stationary milled heads working on a common centre commanding 

 oblique as well as rectangular movements ; the rectangular movements 

 on divided silver plates for recording positions ; and complete rota- 

 tion can be secured, either by hand or by rack and pinion, and can 

 be at any point clamped. 



The sub-stage has rectangular mechanical movements controlled 



