84 SELECTION AND USE 



The inside of all draw-tubes and bodies should be well 

 blackened. When bright or white the glare greatly injures the 

 defining power. When draw-tubes are so arranged that they 

 rub against the inside of the tube forming the body, they in- 

 variably 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 furnished with the Society screw, as 

 by this means it is sometimes possible to use objectives of 

 greater working distance than could otherwise be employed. 



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. 



