CHAPTER I. 



OPTICAL PRINCIPLES OF THE MICROSCOPE. 



1. ALL Microscopes in ordinary use, whether simple or com- 

 pound, depend for their magnifying power on that influence 

 exerted by lenses in altering the course of the rays of light pass- 

 ing through them, which is termed refraction. 1 This influence 

 takes place in accordance with the two following laws, which 

 are fully explained and illustrated in every elementary treatise 

 on optics. 2 



1. A ray of light passing from a rarer into a denser medium, 

 is refracted towards a line drawn perpendicularly to the plane 

 which divides them ; and vice versa. 



II. The sines of the angles of incidence and refraction (that is, 

 of the angles which the ray makes with the perpendicular before 

 and after its refraction) bear to one another a constant ratio for 

 each substance, which is known as its index of refraction. 



It follows from the first of these laws, that a ray of light enter- 

 ing any denser medium perpendicularly, undergoes no refraction, 

 but continues in its straight course ; and from the second, that 

 the rays nearest the perpendicular are refracted less than those 

 more distant from it. The " index of refraction" is determined 

 for different substances, by the amount of the refractive influence 

 which they exert upon rays passing into them, not from air, but 

 from a vacuum; and in expressing it, the sine of the angle of 

 refraction is considered as the unit, to which that of the angle of 

 incidence bears a fixed relation. Thus when we say that the 

 " index of refraction" of Water is 1-336, we mean that the sine 

 of the angle of incidence of a ray passing into water from a 

 vacuum, is to that of the angle of refraction, as 1*336 to 1, or 

 almost exactly as 1J to 1, or as 4 to 3. And thus, the angle of 

 incidence being given, that of the angle of refraction may be 

 found by dividing it by the index of refraction. 



2. On the other hand, when a ray emerges from a dense 

 medium into a rare one, it is bent /row the perpendicular, accord- 



1 It is not considered necessary in the present Treatise, to describe the reflecting 

 Microscope of Amici; since this, although superior to the Microscopes in use previously 

 to its introduction, has been completely superseded by the application of the Achromatic 

 principle to the ordinary Microscope. 



2 See especially Dr. Golding Bird's "Manual of Natural Philosophy,'' Chap. XXII. 



