FUNCTION AND PARTS OF THE MICROSCOPE. 17 



take the direction indicated on the left-hand side, more 

 peripheral rays will enter the lens, and more detailed images 

 will be produced. Such an immersion objective was first/ 

 suggested by Amici in 1850, and next to the achromatic 

 objective this may be considered the greatest single step 

 in the improvement of the compound microscope. 



The degree to which a lens admits the peripheral rays 

 of light is designated by the term angular aperture, which 

 signifies the angle contained between the most divergent 

 rays passing through the objective from the axial part 

 of an object (a point situated on the principal axis of 

 the lens). Obviously the angle will increase with the 

 convexity of the lens and its consequent short focal dis- 

 tance ; with the same lens, the angle will be greater when 

 some homogeneous immersion substance is used. Taking 

 this factor into account, the power of an objective to 

 collect and utilize divergent light-rays is called its Numer- 

 ical Aperture; this quantity is equal to the index of re- 

 fraction of the medium in front of the lens multiplied by 

 the sine of half the angle of aperture. 



12. The Mechanical Parts of the Microscope. — The 

 microscope consists in its essentials of the two systems of 

 lenses of which we have spoken. For steadiness these 

 optical parts must be mounted ilpon a rigid stand; and 

 in addition apparatus is needed for throwing light upon 

 the object to be examined and for focusing, or so ad- 

 justing the relation between the object and the lenses 

 that a clear image may be produced. 



A microscope of the ordinary American pattern, illus- 

 trated in Fig. 15, has a heavy hors&shoe-shaped base 



