256 
Transactions of the Society. 
VI . — The Hackwork Coarse- Adjustment. 
By Edward M. Nelson, Pres. R.M.S. 
( Taken as read, March loth, 1899.) 
A Microscope not fitted with a rackwork coarse-adjustment makes 
only an indifferent instrument, and one which in this country is 
rapidly disappearing from the Microscope world. The quality of the 
coarse-adjustment is an important point in a Microscope, and the 
microscopist will appreciate this importance more and more as his 
technical skill in the manipulation of the instrument improves. A 
tyro nervously brings his low-power objective by means of the rack- 
work coarse-adjustment to within 1/8 of an inch of the focus, and then 
finishes up with the fine-adjustment, whereas an expert microscopist 
would not use the fine-adjustment at all in finding the focus of even 
the widest angled high-power oil-immersion. 
After the focus has been found, a beginner will use his fine -adjust- 
ment in the examination of objects with low powers, while an expert 
would depend solely on the rackwork coarse-adjustment for work with 
medium powers, such as 2/3, 1/2, 4/10 and 1/3 objectives; conse- 
quently, it becomes a matter of importance in the construction of a 
Microscope that the rackwork coarse-adjustment should be of first- 
rate quality. 
History. 
Rackwork was first applied to the coarse-adjustment of the Micro- 
scope by P. Bonannus in 1691 ; this, as might have been expected, was 
of a very crude type (fig. 65), reminding one more of the machinery 
Fig. 65. Fig. 66. 
We find no improvement in the rackwork until 
Benjamin Martin, 1765-1770, introduced the plan of cutting the teeth 
of the rack in a slotted bar (fig. 66). The rack was placed on the inside 
