THE COMPOUND MICROSCOPE 



microscope is to make objects appear larger than 

 they do to the naked eye. When we buy our pocket 

 lens we shall find that these little instruments are 

 constructed to give different degrees of enlargement, 

 some make objects appear five times larger than they 

 do to the naked eye, some ten, some fifteen and 

 some twenty times larger. Twenty times is about 

 the limit of magnification for the ordinary pocket 

 lens. If we are observant we shall notice something 

 else — the greater the magnification the nearer we 



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must hold the lens to our object. Within certain ^(rv\P' 

 limits, this is not a very serious matter, but a point 

 is reached where we must hold our lens so near to 

 the object that we cannot see it, and that is why 

 we cannot obtain very great enlargement with a 

 pocket lens. Despite this fact, as we read in our 

 opening chapter, some very wonderful discoveries 

 have been made with these simple microscopes. 



Now we wish to show how a compound microscope 

 works and, having done so, to explain the uses of 

 its various parts. We shall consider the lenses of 

 the instrument to be double convex ; we do this for 

 the sake of simplicity. Even in the cheapest com- 

 pound microscopes of to-day simple convex lenses 

 are never used, for the reason we explained in our 

 last chapter. To understand the course of the light 

 rays passing through our microscope, however, we 

 may look upon the lenses as being merely double 

 convex. 



Let us try a simple experiment first of all. For 

 the purpose we require two double convex lenses, 



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