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 Compound, 

 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 F, but will be refracted towards the line c c' drawn perpendicularly 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 G is 

 termed the ' angle of incidence;' whilst the angles w o c', GO c', and D 

 o c' are the 'angles of refraction.' And whether the angle of incidence 

 be large or small, its sine e e' bears a constant ratio in each case to the 

 sine w w' or g g' or d d' } of the angle of refraction; and this ratio is 

 what is termed the ' index of refraction.' 



The ' index of refraction ' is determined for different media 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 e e' of the 

 angle of incidence E o c of a ray passing into water from a vacuum, is to 

 the sine w w' of the angle of refraction w o c', as 1.336 to 1, or almost 



