128 PHYSIOLOGY FOR NURSES 



Light, however, does not travel in a straight line when 

 it passes from a medium of one density to that of a 

 greater density. If one places a straight stick in a glass 

 of water, no matter how clear, the stick appears to be 

 bent. This bending of the light rays is called refraction. 

 When light passes into the eyeball, it is leaving a me- 

 dium of little density the air and passing through sub- 

 stances of greater density, the structures in the eye- 

 ball in front of the retina. 



If light is transmitted through a doubly convex, trans- 

 parent body, the rays will be bent in such a way that 

 behind the body they will all come together at one 

 point. This is focusing the rays. The rays of light 

 travel on lines parallel with one another; and are prac- 

 tically all parallel when emitted from objects at a dis- 

 tance of twenty feet. If the lens, the convex transpar- 

 ent body, be of uniform curvature on both surfaces, the 

 ray which passes through the exact center will be per- 

 fectly straight ; but those which enter above will be bent 

 down, those from below, up ; and those from each side, 

 towards the central ray. Hence they will all be thrown 

 on the intercepting surface as a cone whose apex is the 

 point behind the lens corresponding to the bending or 

 refracting power of that lens, and whose base will be 

 the back of the lens. If, however, the object from which 

 the light is reflected be too near the front of the lens, 

 the rays are not parallel, will not strike the lens at the 

 same angle, and will be unequally bent and throw a 

 blurred and indistinct image on the intercepting sur- 

 face. The distance between the lens and receiving sur- 

 face at which the point of the light cone is intercepted 

 is the main focus. The distance between the front of 

 the lens and the illuminated object at which the rays 

 from that object enter the lens as parallel rays is the 



