212 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE 



but injurious to the best work possible to a good instrument. In 

 the interval the swinging sub-stage has been given up, even by its 

 most ardent advocates ; but at the same time in the majority of 

 cases they have abandoned the sub-stage proper and adopted the 

 Continental condenser fitting instead. In fact, the American 

 opticians have chosen almost exclusively, as the basis of their stands 

 of every class, the microscope that has been so long in vogue on 

 the Continent of Europe. 



It will suffice to take examples of the unexceptionally beautiful 

 work of the two leading opticians of America The Bausch and 

 Lomb Optical Company and The Spencer Lens Company. An 

 illustration of the best instrument, known as the * Grand Model,' of 

 the former of these opticians is given in fig. 170. It is designated 

 a ' Continental Microscope,' but is not a mere copy of the best work 

 of Germany or France. The body-tube is large, and the horseshoe 

 base, of Continental fame, is said by the makers to be improved by 

 the ' back claw ' being prolonged ' so as to virtually form a tripod 

 base,' and it is commended as ' extra heavy.' From the figure, how- 

 ever, it would appear to be the extra weight rather than a pro- 

 longed claw that imparts the steadiness. The body is supported 

 on a pillar of two massive columns. The stage is large, and rotal<>> 

 with centring screws. 'The heads of the centring screws are 

 provided with graduations and index, and with a series of line.s 

 recording the number of revolutions of the screw,' so that the 

 position of any given object may be recorded and thus be referred t< > 

 again if the microscope should have been used for other work in the 

 interval. The mechanical stage is worked by one milled head at 

 the side and the other at the top of the stage, the latter position (a^ 

 we pointed out in the last edition of this book when referring to the 

 Tolles mechanical stage) being one in which the efficiency of the 

 mechanism is reduced to its lowest value. We have long advocated 

 the adoption of Turrell milled heads as employed in Powell's No. 1 

 stand ; they give the worker power to effect not only rectangular 

 but diagonal movements, and, without displacing the fingers, to 

 work the stage in all directions. We are pleased, as we have 

 pointed out, to note that the eminent firm of Zeiss have adopted 

 these in their best stand (fig. 139). 



The sub-stage is composed of three parts, arranged one above the 

 other. This sub-stage, with the parts separated to show their 

 construction, is presented in fig. 171. The upper part is a ring 

 carrying a removable iris diaphragm,- so arranged as to come 

 directly into contact with the under part of the object slide. The 

 middle section of the sub-stage is movable vertically on the main 

 sub-stage axis, and carries an Abbe condenser of 1-20 N.A., which 

 can be swung laterally to the left of the instrument so as to put it 

 out of optical use ; but on the other hand it can at will be thrown 

 back into position and placed in oil contact with the object slide with- 

 out altering the position of the upper iris diaphragm. The third and 

 lowest section of the sub-stage carries the large iris diaphragm used 

 below the condenser. Thus it is clear that the whole can be used 

 together, or any one of the three sections can be worked separately. 



