THE MICROSCOPE AND ITS PARTS. 7 



from its cost, is its only objectionable feature. It is 

 named after the gentleman who first brought it to 

 the notice of the opticians, and not as it should have 

 been, after Sir David Brewster, its inventor. It con- 

 sists of a sphere of glass with a deep groove cut 

 around its equatorial center, and filled with a black 

 cement, which acts as a diaphragm to arrest certain 

 rays of light whose presence and action would be 

 undesirable, as they would interfere with the formation 

 of a clear and sharply outlined image. 



The reader may be surprised to learn that there are 

 people who do not know how to focus a lens. I have 

 seen such persons take the instrument as if they were 

 afraid of it. They extend it toward the object in a 

 hesitating way, move it about irregularly for a few 

 moments, throw back the head, look cross-eyed, and 

 say, "Oh yes; I see. How beautiful ! And how very 

 queer it looks!" I once offered a lady an opera-glassj 

 which she put to her eyes and never touched the ad-- 

 justment-wheel that alters the length of the tubes and 

 focuses the lenses on the actors. When she returned 

 it she said, "Thank you. I don't like it much; I can 

 see a good deal better without it. " 



To "get the focus" it is not really necessary to close 

 one eye, although that is usually done. If both eyes 

 are open, the one looking through the lens becomes so 

 interested that the other sees nothing; or, if pre- 

 ferred, we may say that the brain becomes so inter- 

 ested in contemplating the image formed- on the retina 

 of the eye examining the magnified object, that it fails 

 to note the retinal impressions of the other. But if 

 one eye must be closed, it can be done, after very little 

 practice, without clapping your hand over it. This 



