146 SELECTION AND USE 



dent, who will find it fully discussed in any work on optics. 

 The general principles may be best explained by a few experi- 

 mental illustrations. 



Take a piece of cardboard about six inches square, and in it 

 punch a hole about half an inch in diameter. If this card be 

 held in front of a wall upon which the sun is shining strongly, 

 we will see the shadow of the card and a round spot of light 

 exactly the size of the hole. If the card be now moved away 

 from the wall, the shadow and the bright spot will still remain 

 of the same size, showing clearly that the rays proceeding from 

 the sun are sensibly parallel. The same holds true of a bright 

 cloud or a white wall placed at a great distance; but when the 

 wall or other reflecting object is very near, the rays no longer 

 possess this character to the same extent. 



If in the first experiment the wall be illuminated by a candle 

 instead of by the sun, it will be found that as the card is moved 

 from the wall the shadow and the spot become larger, showing 

 that the rays are divergent instead of parallel. The same effect 

 is produced by fixing both the lamp and the card on a stand 

 and moving them away from the wall. 



Convergent rays, that is rays that tend to meet at a point, 

 can be obtained only by passing parallel or divergent rays 

 through a lens, or reflecting them from a concave mirror. By 

 carefully arranging a large convex lens in the path of rays that 

 are divergent, it is easy to render them parallel. They are 

 known to be parallel when the bright spot which they make on 

 a fixed surface, after passing through a hole, is not varied in 

 size by changing the position of the hole. 



The variations which are produced in the appearances of 

 objects when they are viewed by light possessing these different 

 characteristics c^in only be learned by practice, and the young 

 microscopist should experiment in every conceivable way. 



Whatever be the source of light employed, most objects may 

 be viewed by means of any one of several very different 

 methods. Thus, an object, if transparent, may be viewed by 

 transmitted light, thati-s, by light reflected from the mirror, and 

 passing through the object. Ir opaque, it may be viewed by re- 

 flected\ig\iiy in which case the light that passes to the eye through 

 the microscope is reflected from the surface of the object, 



