22 A TYPICAL VERTEBRATE EYE: THE HUMAN 



fibers reflect no light and consequently are invisible in the living lens. 

 The fibrous structure of the lens simply disappears as does a glass bead 

 when dropped into a vial of oil of the same optical density as the glass. 



(B) Optics and Accommodation 



Refraction — The property of substances which is called their 'optical 

 density' has been alluded to above. The higher the optical density of a 

 material, the slower light is able to travel through it. Light travels fastest 

 through a vacuum and very nearly as fast through air, so that for prac- 

 tical purposes the speed of light in air is taken as the maximum. This 

 speed divided by the speed of light in a given substance gives a figure 



Fig. 8 — An analogy for the refractive bending of light rays by a glass plate (see text). 



which is called the 'index of refraction' of that substance referred to air 

 as a standard. 



The effect of the optical density of a substance is to produce a bend- 

 ing of a beam of light which enters that substance at an angle, having 

 previously traversed a substance of different optical density. The amount 

 of the bend in the light-beam will depend upon the difference in optical 

 density of the two substances and upon the angle at which the beam 

 approaches their interface. The direction of bending will depend upon 

 whether the second substance traversed has a higher or lower density, 

 or index of refraction, than the first. 



This bending of light rays when they pass through boundary surfaces 

 is called 'refraction'. Its basis may be best understood if we use an old 



