THE COMPOUND MICROSCOPE 



to the right or left, so that the reflected light rays 

 do not pass directly through the object on the stage, 

 but strike it on one side or the other, thereby giving 

 what is known as oblique illumination. 



The cheapest forms of compound microscopes 

 have all the parts we have mentioned, and focussing 

 is carried out by sliding the tube, with its objective 

 and eyepiece, up and down within its holder, in 

 order to bring the objective further from or nearer 

 to the object. 



In more expensive instruments there are further 

 refinements, in fact, on some of the very costly 

 present-day instruments, there are so many appen- 

 dages and appurtenances that it is doubtful whether 

 some of them are not more of a hindrance than a 

 help, at any rate they increase the possibility of 

 trouble by their liability to get out of order. Such 

 microscopes are only of use to very expert workers ; 

 there are, however, a good many additional features 

 to be found on quite moderate-priced instruments, 

 features which are a great help to the microscopist. 



It is obvious that we cannot attain any degree of 

 accuracy in focussing, especially with high magni- 

 fications, when we must perforce raise or lower the 

 tube by hand. To obviate this difficulty, most 

 microscopes are provided with mechanism known 

 as a coarse adjustment; it consists of milled screws 

 at either end of a metal rod; in the centre of the 

 rod there is a little cog-wheel which engages with 

 a row of notches on the tube. By turning the milled 

 screws slightly in either direction, we can impart 



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