THE MICROSCOPE. 



CHAPTER I. 



OPTICAL PRINCIPLES OF THE MICROSCOPE. 



1. Laws of Refraction .'—Spherical and Chromatic Aberration. 



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 passing through 

 them, which is termed Refraction.* This influence takes place in 

 accordance with the two following laws, which are fully explained 

 and illustrated in every elementary treatise on Optics. + 



I. 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. 



Thus the ray E o (Fig. 1) passing from Air into Water, will not 

 go on to P, but will be refracted towards the line c c' drawn per- 

 pendicularly to the surface a b of the water, so as to take the 

 direction o w. If it pass into Glass, it will undergo a greater 

 refraction, so as to take the direction o G. And if it pass into 

 Diamond, the change in its course will be so much greater, that it 

 will take the direction o d. The angle e o c is termed the ' angle 

 of incidence ; ' whilst the angles woe', g o c' and doc' are the 

 1 angles of refraction.' And whether the angle of incidence be 



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

 reflecting Microscope of Amici ; since this, although superior to the Micro- 

 scopes in use previously to its introduction, has been completely super- 

 seded by the application of the Achromatic principle to the ordinary 

 Microscope. 



t See especially "Brooke's Elements of Natural Philosophy," Sixth 

 Edition, Chaps. xvii.-xx. 



B 



