118 SELECTION AND USE 



defining power. When draw-tubes or long eye-pieces are so 

 arranged that they rub against the inside of the tube in which 

 they are inserted, they invariably make the latter bright by 

 friction. They should, therefore, always slide in a collar. 



It is always well to have the lower end of the draw-tube fur- 

 nished with the Society screw, as by this means it is sometimes 

 possible to use objectives of greater working distance than 

 could otherwise be employed, and this arrangement also affords 

 facilities for the use of amplifiers, erectors, etc. 



Adjustments for Focussing. In the cheaper forms of 

 the microscope the adjustment is made directly by hand, one 

 tube sliding within another. In a better class of instruments 

 the objective is brought nearly into position by sliding the 

 body through an outer tube, and then the final adjustment is 

 made by means of a screw or other mechanical means. But in 

 all the best microscopes, the coarse adjustment, as it is called, 

 is made by means of a rack and pinion, while the fine adjust- 

 ment is made in the manner just mentioned. Instead of a rack 

 and pinion, a chain is sometimes employed, and the coarse 

 adjustment is also made in some cases by screws of very wide 

 pitch, and similar devices. Nothing, however, can equal a 

 smoothly cut and well-fitted rack and pinion. It is sometimes 

 alleged that the chain is more delicate, but this is not so. We 

 have now in our possession a cheap, but well made microscope, 

 the rack and pinion of which is so delicate, that with it we can 

 focus an objective of an eighth of an inch focal distance with 

 sufficient accuracy for all ordinary purposes. 



For ordinary purposes, especially the work of the physician 

 and medical student, the coarse adjustment may be more easily 

 dispensed with than the fine one, but at the same time it must 

 be remembered that any mode of adjustment in which the body 

 is liable to turn round, is incompatible with the use of many 

 important pieces of apparatus. Thus, for example, any turn- 

 ing of the body interferes with the use of the double nose- 

 piece, the polariscope in its higher applications, Prof. Smith's 

 opaque illuminator, etc. A rack and pinion, or its equivalent, 

 should, therefore, always be chosen, especially as it does not 

 add more than five or six dollars to the cost of the instrument. 



