16 The Microscope. 



for this good reason, that the doing so will go far to 

 prevent injury to your sight. You will perhaps find 

 it easier to do this by making a card-board shade to 

 slip on to the top of the tube ; you can shape it by 

 cutting off its corners, and it will be well to cover it 

 with black cloth or velvet. To the rule of keeping 

 both eyes open, Dr. Carpenter adds another, namely, 

 that of not continuing to observe any longer than you 

 can do so without fatigue ; and he reports in his own 

 case an entire freedom from any injury to his eyes, 

 after twenty-five years of microscopic study. I can 

 add a somewhat similar testimony to the safety to 

 eye-sight of (at least occasional) microscopic occupa- 

 tion, for I cannot remember ever to have felt my eyes 

 in the slightest degree fatigued (much less injured) 

 by the microscope. 



But to return to our unpacking of the instrument. 

 You will find, besides the apparatus now explained, a 

 few other things. There will be the " diaphragm," 

 a metal plate perforated with three or four holes of 

 different sizes, and constructed to turn on a pivot, for 

 the purpose of modifying the light from the mirror, or 

 shutting it out altogether when an opaque object is 

 examined. There may also be, packed into the box, 

 two or more bright little concave mirrors, called Lie- 

 berkuhns (from the inventor's name), and intended 

 for the illumination of opaque objects. They fit on to 

 the object-glasses, collect light from the mirror below, 

 and bend it down on the object. A little black cell, 

 to fit somewhere below the stage, is sometimes sup- 

 plied with them, to cut off direct light from the mirror. 



