62 PRACTICAL MICROSCOPY. 



show their action upon various rays of light. An illustration 

 of the bull's-eye condenser is given at Fig. 36, from which 

 it may be understood that it is really a plano-convex lens of 

 crown glass, but mounted in various ways, and the student 

 should remember, on purchasing one, to select such a form 

 as may be turned and twisted in every desirable direction. 



There are many uses for the bull's-eye condenser. It is 

 an indispensable instrument for the illumination of opaque 

 objects ; with it the microscopist is enabled to throw 

 parallel rays on to his mirror from the lamp ; he may use 

 it as a spot lens by fixing a small disc of dead-black paper 

 on the flat surface of the lens, and as an Amici prism for 

 the resolution of diatoms, Nobert's test lines, &c. 



A smaller condenser, generally called a "condensing 

 lens " or " stage condenser " is supplied with some micro- 

 scopes, but its use is very limited, and the student is 

 advised to purchase only the independent stand condenser. 

 With the higher powers a more intense illumination can be 

 obtained by a combination of two condensers, the correct 

 method of using which may be made out by inspection of 

 the diagrams in the next chapter. 



As to quantity of light, we have always found that 

 students have too much of it, and therefore it may be as 

 well to consider the diaphragm next. The diaphragm is 

 a thin plate of metal perforated with holes of various 

 shapes and sizes, and is used in order to cut off the super- 

 fluous rays from an object, or leading to an object, under 

 examination. At one time they used to be placed at 

 considerable distances beneath the object, but the practice 

 nowadays is to put them immediately beneath the slide, 

 which doubtless is their proper place. 



The ordinary form is shown at Fig. 37, and is called a 

 wheel of diaphragms ; it is usually supplied to all student's 

 and third-class instruments. 



