FUNCTION AND PARTS OF THE MICROSCOPE. ‘7 
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 upon 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 horseshoe-shaped base 
