THREE GREAT TYPES OF MICROSCOPE 



199 



was, however, eventually cast in one piece, which gave it a solidity 

 which the former did not possess. 



The introduction of the Lister limb brought its inevitable 

 troubles notably, with the fine adjustment to which we have fully 

 referred under that head. But in the Ross-Zentmayer model, a 

 later form, the body and the coarse adjustment were both carried by 

 the fine-adjustment lever and screw. 



This form could not as it did not long prevail. Its existence 

 \vas ephemeral, and in its place was put a modification of the form 

 devised by Zeiitmayer, known subsequently as the Ross-Zentmayer 

 model. This was the Ross- Jackson instrument with a ' swinging 

 sub-stage.' This instrument is illustrated in fig. 161. It will be 

 seen that the foot is a true tripod, consisting of a triangular base 

 with two pillars rising from a cross-piece, which carried the trun- 

 nions. 



Here it may be as well to point out the differences which exist 

 between the three great types of microscope, viz. the bar move- 

 ment, the Lister limb, and the Jackson limb. In the bar movement 

 we find a transverse bar uniting the lower end of the body to the 

 coarse adjustment bar (figs. 157, 159). In the Lister the body is 

 supported through a greater or less portion of its entire length, 

 the limb being formed of one solid casting (figs. 160, 161, 162, 167). 

 In the Jackson the dovetailed groove which carries the sub-stage 

 slide is included in the casting, and the groove for the coarse ad- 

 justment of the body, as well as that for the sub-stage, is ploughed in 

 one cut (fig. 165). Jackson also designed the double pillar foot 

 (fig. 161). 



We have already assessed the value of a swinging sub-stage and 

 found that in our judgment it is at best redundant and really 

 adverse to the accomplishment of the best scientific work. 1 No 

 microscope is complete without a good condenser ; all and much more 

 than all that can be done by a swinging sub-stage can be done with 

 a slotted stop at the back of the condenser. This elaborate appen- 

 dage is therefore without justification. Yet in the impatience for 

 large illuminating apertures, which were not at that time provided by 

 condensers, this phase of pseudo-illumination was carried to a still 

 greater and more elaborate development in the production of a con- 

 centric microscope. This was a Ross-Wenham, known as the radial 

 microscope. But elaborate and costly as it was it never justified its 

 existence, and like the whole group of * concentric ' and * radial ' 

 microscopes, it has passed away simultaneously with the abolition of 

 ' oblique illumination,' and is to-day a not very interesting curiosity 

 in the history of the modern microscope. 



A large and extremely well-finished stand is made by Messrs. 

 Watson, known as the Yan Heurck microscope in its best form : it is 

 illustrated in fig. 162. The body has two draw tubes, one of which 

 is actuated by rack and pinion, and the other sliding inside it so 

 that a range of body length varying from 142 mm. to 300 mm. can 

 be obtained. The coarse and fine adjustments have very wide 

 bearings, and the exact relationship of the pinion to the rackwork 



1 P. 188 et seq. 



