FUNCTION AND PARTS OF THE MICROSCOPE. 3 
2. Function of the Microscope.—In order that a visual 
image may be produced, a certain definite area of the’ 
retina must be stimulated, and objects whose images are 
smaller than this minimum will ordinarily be invisible. 
Normal vision is therefore limited to fairly large objects 
near at hand. If, however, the rays proceeding from 
more remote, or smaller, objects can be collected by a 
lens and bent so as to produce a larger image, the objects 
may be seen. This end is attained by the telescope and 
the microscope,—the former producing enlarged images 
of large and remote objects, the latter producing enlarged 
images of near and minute objects. 
3. Laws of Refraction.—The formation of such 
images by the microscope is dependent upon the fact 
that rays of light in passing from any medium to one 
of different density experience a change in direction, 
unless they impinge at right angles to the surface of 
contact. 
The deflection thus produced has been compared to 
the alteration in the course of a column of troops on pass- 
ing from a smooth parade-ground into a ploughed field. 
If, in Fig. 2, aba,b, represents a body of troops march- 
ing in the direction indicated by the arrow, it is apparent 
that the men on the right of the line will reach the 
ploughed field 6,a,0,¢, first and will be somewhat re- 
tarded. The left of the line will gain upon the right and 
the column as a whole will execute a partial right face. 
In passing out of the field on the other side, the right 
of the line will reach the smooth ground first and gain 
as much as it lost before, the final result being that the 
