The Hackwork Coarse- Adjustment. By E. M. Nelson. 261 
The question now arises : — Is there any better form of rackwork 
possible ? This, I believe, can be answered in the affirmative. The 
form that might be advantageously adopted is that known as a stepped 
rack (not of the diagonal but of the ordinary straight type). The 
various conditions enumerated above should be rigidly adhered to, but 
the number of teeth in the pinion might usefully be increased to 
fourteen. In stepped rackwork two parallel racks engage in the same 
pinion ; one rack, however, is placed so that its teeth are stepped an 
amount equal to the “ back lash ” behind those of the other, i.e. 1/11 
of the pitch. 
The racks should be cut together, and fixed in the position they 
were cut. The object of this plan is that one of the racks shall be 
in action when the bar is racked up, the other when it is racked down ; 
so, then, if the racks are properly placed relatively to one another, 
“ loss of time ” is simply an impossibility ; and, be it observed, this 
desirable result is obtained without forcing the teeth of the pinion 
into those of the rack, and without increasing the engaging friction, 
provided that the shape of the teeth is true and the formula accurately 
followed. This plan would be well worth a trial. 
Diagonal stepped rackwork was suggested by the late J. Mayall, 
Jun., who went so far as to propose three diagonal racks * ; but it is 
doubtful if stepping diagonal rackwork is not rather disadvantageous 
than otherwise. It would seem that Mr. Mayall missed the point of 
the problem ; for he stepped the pinions as well as the racks. In 
his three-rack system the teeth of the racks and pinions are dis- 
placed 1/3 of the pitch behind one another, and all the three pinions 
drive at the same time ; this of course adds greatly to the strength 
of the mechanism, and it would be very serviceable in mill work, for 
example, in those cases where it was necessary to transmit great power 
with a small-sized pinion. 
In the Microscope, however, the work to be done is light, and the 
strength of either the leaves of the pinions or the teeth of the rack 
does not enter into the question, as there can be no possible risk of 
stripping. The essence of the problem before us is how to get rid of 
“ loss of time,” and yet keep the amount of engaging friction at a 
minimum. 
Now, engaging friction with a pinion of eight leaves is much greater 
than with a pinion of sixteen. By stepping two racks and two pinions 
of eight, you may obtain, by Mr. MayalTs plan, the strength of a 
pinion of sixteen, because you have doubled the number of teeth acting 
at the same time, but obviously you also largely increase the amount 
of engaging friction. 
Now, as the strength of the material in ordinary Microscope con- 
structions is ample for the work required, it goes without saying that 
the search after some method whereby the mechanism is to be 
* Journ. R.M.S., vol. iv. ser. 2, 1884, p. 958, ligs. 153, 154. 
