FOCUSSING ARRANGEMENTS 



16 



this firm are as good after thirty years' use as they were when 

 new T . 



Frequently bad workmanship is concealed by the free employment 

 of what is known as * optician's grease ' and an over-tightening of the 

 pinion, driving its teeth into the rack, which, of course, speedily 

 ends in disaster. 



If we desire to practically test this part of a microscope, we 

 must remove the pinion, take out .the bar, clean off the ' optician's 

 grease ' w r ith petroleum from both bar and groove, oil with watch- 

 maker's oil, and replace the bar in the groove, and before refixing 

 the pinion see if it slides smoothly and without lateral shake. 



What has been said about the ' springing ' of the bar in this special 

 instance applies equally to all moving parts, in stage and sub-stage 

 movements, and wherever constant friction is incurred ; equally 

 applicable, too, is the 

 lubricant we suggest. 

 An instrument left 

 unused in its native 

 * grease ' for twelve 

 months becomes so im- 

 mobile in most of its 

 parts by the hardening 

 of its * normal ' lubri- 

 cant that motion be- 

 comes a peril to its future 

 if persisted in in that 

 condition. 



If a ' coarse adjust- 



ment 'be what it should FlG 124A ._ Nelson , s , stepped , rack> invented in 1899 . 

 be, all lower powers 



should be exclusively and perfectly focussed by it, and with the 

 highest powers objects should be found and focussed up to the point 

 of clear visibility. 



The exceedingly useful method of ' diagonal rack and twisted 

 pinion' was introduced by Messrs. Swift and Son about 1880 and 

 has since been universally adopted. Its mode of operation is seen 

 in fig. 124, a sectional drawing of this part of one of Swift's micro- 

 scopes. 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 expedient there is more gearing contact between rack and 

 pinion, which prevents ' loss of time ' and obviates the necessity for 

 unduly forcing the teeth of this pinion into those of the rack. 



Mr. Nelson has had made by Messrs. Watson and Sons a still 

 better form of rackwork. It is what is called a 'stepped' rack (not 

 of the diagonal, but of the straight type). In this very admirable 

 form two parallel racks engage in the same pinion ; one rack, how- 

 ever, is placed so that its teeth are stepped an amount equal to the 

 ' back-lash ' behind those of the other, e.g. r l r of the pitch. 



These racks have to 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, and the other when it is 



M 



