Arrangements for focussing. 25 



exists, it should be fitted in a workmanlike manner, and a 

 proper support given to the fitting when in the optical axis 

 to make it perfectly rigid. 



The choice with regard to a sub-stage would therefore be 



In a first-class microscope : Hackwork and centring 

 adjustment and fine adjustment ; rackwork rotation, if for 

 examination of crystals or petrology. 



Second-class instrument : Eackwork and centring adjust- 

 ments, and means of lifting aside out of the optical axis. 



, Student's instrument : The same as the second-class or 

 the plain tube fitting. 



ARRANGEMENTS FOR FOCUSSING. 



The next point for consideration is the means of 

 focussing the object-glass. This is done by two adjust- 

 ments, called the coarse and the fine movements. The 

 former consists of a rack and pinion actuating the body 

 in a very true-fitting dovetailed bearing as per Fig. 3, 

 shown on page 24. In the illustration it will be seen that 

 the rack is cut diagonally, and this undoubtedly, although 

 rather troublesome in the making, is the best kind, a softer 

 motion "being obtainable than with the horizontal form. In 

 order that it may work at its best, each tooth of the rack 

 should be carefully ' ground in ' that is, fitted to a leaf 

 in the pinion and so that the fitted tooth of the rack may 

 always engage the right tooth of the pinion, it is necessary 

 to so fix the body that when racked up as high as possible 

 it may not be withdrawn from its bearings and rackwork, it 

 being, in fact, provided with a ' stop ' screw. It has been 

 recommended that microscopists should take their instru- 

 ments to pieces in order that they may judge of their 

 workmanship ; but in reality a well-made microscope 

 requires as carefully putting together as a watch, and for a 

 novice to attempt to undo the parts means very probable 

 detriment to the instrument, and the name of a first-class 

 maker is a guarantee of the good work, without which he 

 could not possibly maintain his reputation. 



