258 
Transactions of the Society. 
the pinion was mounted in a spring bearing (fig. 68). The springing 
of the pinion was a great advance, and made this the best coarse-adjust- 
ment that had been applied to the Microscope up to that time. Yarley 
drilled holes in the edge of the milled head of the coarse-adjustment, 
and fitted capstan bars to them for purposes of fine-adjustment. 
A further improvement in the coarse-adjustment was made by 
Powell, in a Microscope having a wedge fine-adjustment fitted to the 
stage, which - he brought before the Society of Arts in the Session 
1834-35. In this Microscope an equilateral prism bar, with the angles 
much truncated, was substituted for the rectangular bar of Yarley’s 
“ Yial ” Microscope ; the rack, with 22 teeth per inch, was attached to 
the posterior plane side of the bar, and was not sunk ; the bar was also 
sprung.* A coarse-adjustment with a sprung pinion is found in 
J. Smith’s Microscope with the Lister limb, purchased by the Society 
in 1841, and also in Edwin Quekett’s Microscope, both of which are 
in the Society’s cabinet. Edwin Quekett, who was one of the founders 
of this Society, presented his Microscope, which was partly made by 
his own hands. He died in 1847, aged thirty-nine. 
We can pass on to 1861 before we find any further improvement 
in the coarse-adjustment ; in this year Powell brought out his “ No. 1,” 
which, with the sole exception of the stage rotation, is the same as 
you now know it.f The improvement consisted in the shape of the 
bar, which was formed from an isosceles prism, having a base 14 in., 
and side 24, with If in. cut away from the apex, leaving a massive 
trapezoid bar with a base 1J-, top 1 J, and sides 7/10 in. 
Having brought up the history of the coarse-adjustment to a 
recent date, it will be necessary, before proceeding further, to consider 
very briefly the theory of rack-and-pinion work. 
Theory. 
In rack-and-pinion work, such as we have in a coarse-adjustment, 
the pinion is termed the driver, and the rack the follower. 
Let a perpendicular be dropped from the centre of the pinion to 
the rack (see dotted line in fig. 69). Now it is important that the 
teeth of the driver should only come into contact with those of the 
follower past this line towards the right hand, supposing the rack to 
be moving in a direction from left to right. The reason for this is, 
that all friction occurring past this line is called disengaging friction, 
which may be compared to that occasioned by a man walking on a 
pavement trailing his stick after him with the ferrule on the ground ; 
* Transactions Society of Arts, vol. 1. part ii. p. 108, pi. 3 (183G). (Account of 
stage fine-adjustment only given; we know, however, from existing examples, the 
form of Microscope to which this stage was fitted.) Several of the plates illustrating 
the Microscopes were presented to the Society of Arts by Dr. R. H. Solly. Many of 
the plates relating to various kinds of machinery were drawn by C. Yarley, and 
engraved by Edmund Turrell (the inventor of the well-known Microscope stage), 
f Quarterly JoiTrn. Micr. Sei., vol. i. N.S., 1861, p. 173, fig. 
