212 TIIK IIJSToKY AND DEVELOPMENT OF THE MICROSCOPE 



Imt injurious to the best work po-.-ible to a good instrument. In 

 the interval tin- swinging .-ub-.-tage lias been given up, even by its 

 nio.-t ardent ;nlvMc;i1i'> : l>ut ;i1 the same time in the majority of 

 cases tli -V have abandoned the sub-stage proper and adopted the 

 (' nitiiic nial condenser titling instead. In fact, the American 

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

 ot' everv class, the microscope that has been so long in vogue on 

 the Cont incut of Kurope. 



It will sutlice to take examples of the unexceptionally beautiful 

 work of the two leading opticians of America The Bauseh and 

 Lomb Optical Compam and The Spencer Lens Company. An 

 illustration of the best instrument, known as the ' Crand Model/ of 

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

 a ' Continental Micro-cope." 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 b\ 

 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 rotate.- 

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

 provided with graduations and index, and with a series of lines 

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

 position of any given object may be recorded and thus be referred to 

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

 interval. The mechanical stage is worked by one milled head a* 

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

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

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

 mechanism is reduced to its lowe.-t value. We have long advocated 

 the adoption of Turrell milled heads as employed in Powell's Xo. 1 

 stand; they giv< the worker power to effect not only rectangular 

 but diagonal movement.-, and. without displacing the fingers, to 

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

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

 these in their besl .-land (tig. 1,">'.I). 



The sub-stage is compo.-ed oft hive parts, ai-ranged one above the 

 other. This sub-stage, with the part.- 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 vv it h t he under part of the object slide. The 

 middle sect ion of the .-lib -tage is movable vertically on the main 



sill) stage axis, and carries an A I >he condenser of 1 '20 ~N.A., which 



can be swung laterally to the lel'i of the i n.-t rmneiit so as to put it 

 out of optic..] LI . : but on the other hand it can at will be thrown 

 I tack into posit ion and placed iii oil contact vv it h t he object slide with- 

 out altering t he po-it ion oft he upper iii- diaphragm. The third and 

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

 below the condenser. Tim- it i- dear that the whole can be used 

 to^t her. or anv <>i e of the 1 hree sect ions can be worked separately. 



