32 HOW TO SEE WITH THE MICROSCOPE. 



radial arm is placed in position for oblique light the 

 mirror will be nearer the well hole than when the arm 

 is centrally disposed ; therefore, the mirror should be 

 arranged to slide on the arm, so that the proper com- 

 pensation may be effected. 



The mirror should, of course, be well mounted in its- 

 own semicircular arm, so that universal motions may be 

 obtained, and the semicircle should be closely attached 

 to the slide moving on the radial arm; that is to say y 

 there should be no intermediate, short, and jointed arm& 

 or elbows such as are found on many so-called first 

 class instruments, and are a most intolerable nuisance. 

 It is quite possible at times to remove these intermedi- 

 ate arms, and attach the mirrors directly to the radial 

 arm, and at a trifling expense, thus transforming a 

 faulty stand into a really serviceable one. 



Examine the concave mirror carefully, as to its tocal 

 length. To do this, place a piece of white letter paper 

 on the stage, and using a common candle reflect the 

 light on the paper ; now move the mirror up and down 

 on the radial arm, and see if you can get a tolerably 

 well defined image of the flame. It may occur the 

 radial arm is too short for the focal length of the 

 mirror, and that it would be impossible to lengthen it 

 sufficiently; this being the case, reject the stand. 



One would naturally suppose that an item like the 

 last would surely be attended to on the part of the 

 maker; nevertheless, many stands are made and sold 

 with glaring defects of this character. 



At the risk of being tiresome, let me especially insist 



