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 wiU 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, ahafii 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 b^a^b^a, 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 Hne will reach the smooth ground first and gain 

 as much as it lost before, the final result being that the 



