THE MICROSCOPE. II 



/ as the centre, and drawing lines from the perpendicular 

 at right angles with it to the points at which the circle 

 intersects the incident and refracted rays. Thus a b and 

 c d are the sines of the angles of incidence and refraction 

 respectively. In the case of air and water the sines are 

 as 4:3. The refractive index of air is so small, that for 

 practical purposes its refractive power is discarded, and 

 therefore i=i'333 the refractive index of water. The 

 following table shows the indices of refraction for various 

 substances, many of which are used in practical histology : 



INDICES OF REFRACTION. 



r 



*~ 



Diamond . . 2*44 



Flint glass 



Crown glass 



Canada balsam 



Clove oil 



Oil of turpentine 



Glycerine 



Alcohol 



Aqueous humour 



Pure water 



Air ... 



As will afterwards be seen, such substances as Canada 

 balsam, clove oil, turpentine, and glycerine, having a com- 

 paratively high refractive index, are constantly employed 

 in histology, for the purpose of increasing the transparency 

 of tissues. 



22. Lenses. If a ray pass obliquely through a medium 

 with parallel faces (Fig. 6), the excident assumes the same 

 direction as the incident ray, but, if the faces be not par- 

 allel, the excident ray assumes a new direction. Thus, 

 let r (Fig. 8) be a ray impinging on a triangular prism at a, 

 it bends towards the perpendicular /, and on emerging at 

 b, bends from it. The excident ray r thus takes up a 

 direction entirely different from ;-. If two prisms be joined 

 by their bases (Fig. 9), the rays A B, being refracted as in- 

 dicated in Fig. 8, are rendered convergent and brought to 



