The Sub-Stage 21 



ARRANGEMENTS FOR FOCUSING. 



The next point for consideration is the means of focusing 

 the object-glass. This is done by two adjustments, 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 19. 

 In the illustration it will be seen that the rack is cut 

 diagonally, and this is undoubtedly 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 has to 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 correct leaf of the pinion, it is 

 necessary so to 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 as possible, it may not be withdrawn from its bearings and 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. 

 The pinion should have suitable provision by means of 

 adjusting screws for exactly controlling the stiffness of the 

 rackwork action and for taking up slight backlash which 

 may arise in consequence of wear and tear. An illustration 

 of the method adopted to secure this result is shown in 

 Fig. 4, page 19, in which the pinion P is held in position by 

 a block of metal, N, against which pressure is exerted by 

 two screws, one of which, M, is shown in the figure. With 

 this arrangement the most exact relation of pinion to rack 

 can be established and maintained. 



It has been recommended that microscopists should take 

 their instruments to pieces in order that they may judge of 

 their workmanship ; but in reality a well-made microscope 

 requires to be as carefully put together as a watch, and for 

 a novice to attempt to undo the parts means very probable 

 detriment to the instrument. The name of a first-class 

 maker on an instrument may generally be considered a 

 guarantee of good workmanship, otherwise he could not 

 possibly maintain his reputation. 



Some cheap students' microscopes, instead of being pro- 



