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Transactions of the Society. 
strengthened by the increase of engaging friction is simply labour 
lost. In my system, although there are two racks, only one of them 
is in action at a time, consequently there can be no increase of the 
engaging friction. I am not aware that stepped rackwork arranged in 
this manner has been applied to the Microscope before. 
Synopsis of History. 
Author. 
Nature of Improvement. 
Date. 
P. Bonannus .. .. 
Application of rackwork 
1691 
B. Martin 
Rack in slotted bar 
circa 1765 
G. Adams 
Body-focusser, ordinary rack, rectangular bar . . 
1771 
B. Martin 
Body-focusser, one inch of rack in slot in 
tube (telescope form); example in Society’s 
cabinet 
circa 1776 
B. Martin 
Slutted bar attached to triangular prism ; ex- 
ample in Society’s cabinet 
circa 1780 
Dr. R. C. Goring .. 
Body-focusser. triangular bar racking out of a 
cylindrical rod. bar sprung 
1829 
C. Varley 
Square bar sprung, pinion sprung, rack 40 teeth 
per inch 
1833 
H. Powell .. .. 
Equilateral prism bar with truncated angles, bar 
sprung, pinion sprung, rack 22 teeth per inch. 
1834 
H. Powell .. .. 
Trapezoidal bar sprung, pinion sprung 
Diagonal rack and twisted pinion, bar sprung, 
1861 
J. Swift 
pinion not sprung 
1880 
T. Powell 
Application of diagonal rack and twisted pinion 
to trapezoidal bar, bar and pinion sprung 
1893 
