ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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joint from the top of the pillar to the bottom ; this from our present 
point of view must be considered a retrograde step. 
This Microscope is, as we have already mentioned, a body focusser, 
so also was the Variable,; but the Variable was hampered with a fine 
adjustment movement mounted on the same slide ; therefore a clamping 
nut had to be released to throw the fine adjustment out of action when 
the rackwork coarse adjustment was used. There can be no doubt that 
the suppression of the fine adjustment effected an improvement ; for a 
good rackwork coarse adjustment was quite sufficient to focus any non- 
achromatic objective of that time. 
Fig. 129. 
The optical part of this Microscope was similar to that of the Vari- 
able, and consisted of a double eye-lens, a field-lens, and another lens 
lower down the body, which, as we have stated before, formed the back 
lens of the objective, the alteration in power being effected by changing 
the front lens of the objective only. It is probable that this arrangement 
of the optical part of the instrument was due to Benj. Martin ; though, so 
far as is known, the first publication of it is in the account of the Vari- 
able Microscope. At the end of the nose-piece a dove-tailed slot is seen, 
into which slides a plate containing six powers. This is the first instance 
we have of such a contrivance fitted to a compound Microscope ; but it 
