The Hackwork Coarse- Adjustment. By E. M. Nelson. 257 
of the slot, in order that the inside of the plain bar opposite the rack 
might give a smooth sliding surface for the bearing. He afterwards 
improved this plan by fixing the slotted bar to a stout post ; this would 
give more stability, the slotted bar probably proving too weak for the 
work it was intended to do. 
In the large Benjamin Martin Microscope (1780), in the Society’s 
cabinet, the slotted bar is attached to a triangular prism bar. This 
is specially interesting, because it is the first appearance of a triangular 
prism bar in the Microscope. In 1771, G. Adams brought out his 
“ Variable ” Microscope ; this very interesting model had a rackwork 
coarse-adjustment of a rough type, scarcely equal in efficiency to 
Benjamin Martin’s slotted bar. The slotted bar attached to a stout 
rectangular bar was a favourite form of coarse-adjustment at the close 
of the last and the opening of this century ; it was largely used by 
Adams and Jones ; but it should be remembered that these Microscopes, 
with the exception of the “ Variable,” were stage focussers. 
The next improvement in the coarse-adjustment was designed by 
Dr. Goring, and is to be found in his “ Operative Aplanatic Engiscope,” 
made by Andrew Pritchard (May 1829).* This instrument is a body 
focusser ; an equilateral triangular bar, to which a transverse limb 
carrying the body is attached, racks out of a cylindrical rod. The 
triangular bar has a groove placed in one of its sides, into which is 
brazed a strip of brass with a rack cut on it, thus forming what may 
be called a sunk rack. The bearing of the pinion which actuates this 
rack is seen in fig. 67 ; it should be particularly noticed that springs 
were placed above and below the pinion, to bear on the triangular bar 
on the same side as the rack. This Microscope had, therefore, a 
coarse-adjustment far in advance of any that preceded it. About this 
time, Andrew Pritchard made Microscopes on a less elaborate and 
more practical model than that of Dr. Goring’s, but the coarse- 
adjustments were of a similar pattern to his. In March 1831, Mr. W. 
Valentine designed a Microscope, which was made by Andrew Ross ; 
this had a coarse-adjustment on the same model. It is interesting to 
note, that Dr. Goring, in describing his Microscope, suggests the 
employment of a differential screw for a fine-adjustment.f 
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The next improvement we find is Cornelius Varley’s “ Vial 
Microscope,” which was made by Hugh Powell in 18334 The bar 
was square, and was sprung ; the rack had 40 teeth to the inch, and 
* ‘ Microscopic Illustrations,’ 1st edition (1830), p. 52. t Loc. cit. 
X Transactions Society of Arts, vol. 1. part i. p. 166, pi. 5 (1836). Varley’s 
Microscope was brought before the Society in the session 1833-31. 
