10 



VEGETABLE HISTOLOGY. 



it is called a plane mirror, and when spherical, it is called a 

 spherical mirror. These are the commonest forms, and both 

 are used on microscopes for illuminating objects examined. 



PLANE MIRROR. The manner in which this reflects light is 

 illustrated in Fig. 1. AB is a vertical section of the mirror, 

 CD is a perpendicular to the surface, OD is an incident ray, 

 and the angle ODC is the angle of incidence, DP is the reflected 

 ray and GDP is the angle of reflection. It has been shown 

 experimentally, as well as deduced theoretically from the wave 

 theory of light, that in a plane mirror the reflection follows a 

 fixed law, namely, that the angle of reflection is always equal 

 to the angle of incidence ; that is, the angle GDP is always equal 

 to the angle ODC. 



Fig. 1. 



Fig. 2. 



What is true for one ray, as illustrated in Fig. 1, is true for 

 any number of rays. Hence if a beam of parallel rays of light 

 be reflected from a plane mirror, the angles of reflection of the 

 rays will all be equal, consequently the reflected beam will con- 

 sist of parallel rays. Similarly, by geometrical construction 

 based on the above law, it may be shown that divergent rays, 

 after reflection, remain divergent to the same degree as before 

 reflection. This is shown in Fig. 2. It follows, therefore, that 

 the only function of a plane mirror is to change the path of 

 light that falls upon it. For example, on the microscope it 

 causes a beam of light that is received from a window to be 

 thrown up vertically through the object and the lenses. 



SPHERICAL MIRROR. As indicated by the name, the reflecting 

 surface of this mirror is part of a sphere, and may be either 

 convex or concave, but only the latter is of interest in connec- 

 tion with the microscope. 



In Fig. 3, ABD is a section through the 

 middle point B of the spherical surface, C 

 is the center of curvature. The line GCB 

 through the center, C, and the middle point, 

 B, of the mirror, is called the axis. Apply- 

 ing the law of reflection, it may be shown 

 by geometric construction that, in the case 

 of a spherical surface, any ray AO, parallel 

 to the axis BCG, will be reflected to a point F on the axis. This 

 point is called the principal focus and lies half-way between 

 C and B. It will be seen that a beam of light, consisting of 



Fig. 3. 



