260 
Transactions of the Society. 
We must therefore consider the various methods that are employed 
to get rid of this “loss of time.” The first method, one that has 
been largely employed for the fifty years subsequent to 1829, the date 
of Goring’s improvements, is to wholly disregard the shape of the 
teeth, their pitch and depthing, to select any pinion that will fit any 
rack, and then to force the teeth of the pinion into the rack by screw- 
ing up the top brass of the hearing. Figs. 70 and 71 show the kind of 
bearings fitted to Microscopes in order to carry out this method. In 
fig. 7 0 the faces of the brasses are cut away so as to leave space for the 
tightening of the screws, as the upper brass is worn down by the 
friction of the pinion, which friction, owing to the engaging friction 
of the teeth, must be considerable. In any case fig. 70 is but a poor 
makeshift, and altogether a rickety affair, because the brasses are not 
tightly faced up as in fig. 67. In fig. 71 a somewhat better makeshift 
Fig. 70. 
Fig. 71. 
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is seen, for the edges of one side of the bearing are brought together, 
thereby making a somewhat steadier bearing than that in fig. 70. In 
Microscopes made of hard brass these devices might succeed for a 
time, but, with the present fashionable soft brass Microscopes, in a 
very little while the brasses cut, and then there comes a speedy end 
to their freedom from “ loss of time.” 
The second method, a far better one, is known as the “ diagonal 
rack and twisted pinion.” This was, I believe, introduced by Messrs. 
Swift and Son, about 1880, and is now universally adopted.* 
The advantages gained by this method are due to the twist in 
the pinion being a shade steeper than the diagonal of the rack, by 
which simple expedient not only is “loss of time” prevented, but 
all necessity for unduly forcing the teeth of the pinion into those of 
the rack is obviated. 
The bearings commonly used with diagonal rack work are of the 
types shown in figs. 67, 70, and 71 ; but 1 got Powell to put diagonal 
rackwork to my Microscope (his No. 1) with such satisfactory results 
that he has permanently adopted it. Diagonal rackwork used in con- 
nection with his prism-bar and sprung-bearing, fig. 68, must be 
regarded as the highest type of coarse-adjustment at the present 
time. 
There is one drawback to the employment of diagonal rackwork, 
viz. that it throws end-pressure on one side of the pinion ; this in the 
Microscope is, however, a small matter. 
* Journ. R.M.S., vol. i. ser. 2, 1881, p. 518, fig. 121. It has been said that the 
application of this method to the Microscope dates back to Andrew Pritchard’s time ; 
but this statement I am unable to verify. 
