204 TH E HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



by their invention of the vertical lever fine adjustment (figs. 133 and 

 135) Swift and Son have made possible a useful future for the Lister 

 limb ; and their model of this form is shown in fig. 166, known as 

 the ' Best " Challenge " Microscope.' It has a beautifully made coarse 

 adjustment, the special fine adjustment invented by this firm, a cir- 

 cular rotating stage moved by rack and pinion or by hand, and is 

 provided with divided silver plates to the rectangular movement. 

 The sub-stage is complete for centring as well as focussing, and has 

 rotary movement for use with polar i scope. The stand is a firm 

 form of tripod, and the mirrors are well worked and mounted on a 

 double crank. 



All the movable parts of Swift's instruments are sprung on Powell 

 and Lealand's method, and the movements are smooth and sound. 

 Many stands had been devised by American opticians up to the time 

 of the publication of our last edition of this work, but they were 

 based upon one or other of the great English models, and the modi- 

 fications, whether for good or evil, were adopted into the then modi- 

 fications of the older English types, and were incidentally described. 

 It should be remembered that Zentmayer, of Philadelphia, devised 

 the model from which the Ross-Zentmayer was finally formed. Its 

 principal feature was to obtain oblique illumination in one azimuth 

 by the swinging stage which we have emphatically shown in this, as 

 we did in the last edition, to be a pernicious adjunct for practical 

 purposes. The fine adjustment of this instrument was most defec- 

 tive. Tolles, again, who wholly deserves the very high reputation he 

 attained, made an instrument in which he mounted the stage on a 

 disc ; near the edge of this disc the sub-stage is made to travel in a 

 groove carrying the condenser, or dry combination, in an arc round 

 the object as a centre. This was only another elaboration of the 

 same swinging sub-stage. 



In later constructions of this form, Tolles first used the mechanical 

 stage actuated by two pinions vertical to the surface of the stage, 

 and subsequently adapted by Ross. The fine adjustment in this 

 instrument had the fatal defects characteristic of its form. 



Bulloch, another American maker of note, made some modifica- 

 tions in the Zentmayer model, but they were in the interests of the 

 swinging sub-stage, and, although no doubt ingenious, must pass 

 with this transient form of the microscope. 



A modification of this stand was devised by Bulloch ; it presents 

 no special point, save the employment of a Gillett condenser with 

 the diaphragm drum above the lenses ! 



A later development of this form of instrument by the same 

 maker was made some years after, but the chief difference consists 

 in the adoption of a stage in which the milled heads stand upon 

 the stage, which is the reverse of an advance. Since, however, the 

 swinging sub-stage form of instrument has been entirely superseded, 

 American makers have adopted, with very slight modifications in 

 form, none in principle, the Continental stand, which is made with 

 admirable precision and conscientious care, but still retains its chief 

 features. It may therefore be of service to consider the principal 

 recent modifications of the Continental stand, so that they may be 



