APPENDIX A 

 THE MICROSCOPE AND ITS OPTICAL PRINCIPLES 



For an understanding of the optical principles involved in 

 microscopy, four things must be borne in mind with regard to a 

 ray of ordinary daylight: 



1. It has an appreciable breadth. 



2. It travels in a straight line in a homogeneous medium. 



8. It is bent (refracted) in passing obliquely from one medium 

 into another of different density. 



4. It is in reality a composite of a number of different colored 

 rays, ranging from violet to red, and each of these has a different 

 refrangibility. 



The amount of refraction undergone by light in a given case 

 depends upon the difference in density of the two media which 

 the light traverses. Thus, glass is denser than 

 air, hence, in passing from air obliquely through 

 a glass plate (Fig. 41), a ray of light A B would 

 be bent out of its original course. On reaching 

 the air again, however, it would resume its 

 original direction, although it would be dis- 

 placed to an amount equal to the distance A , 

 between A and A ' . It is on account of such 

 displacement that an object in water, for ex- Fie.4i. 



ample, appears to be at a different point from where it really is. 

 On the other hand, after traversing a prism, a ray does not 

 resume its former direction, but takes a new course upon leav- 

 ing as well as upon entering the prism (Fig. 

 42). This new direction is always toward 

 the base of the prism, and the amount of de- 

 viation depends upon the shape and density 

 of the prism. If the base is down, then the 

 FIG. 42. ray is bent downward ; if the apex is down, 



the ray still deviates towards the base, that is, it is bent upward 



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