522 THE HUMAN BODY. 



indices of those media are the air, 1; the aqueous humor, 

 1.3379; the lens (average), 1.4545; the vitreous humor, 

 1.3379. From the laws of the refraction of light it therefore 

 follows that (Fig. 149) the rays C d will at the corneal surface 

 be refracted towards the normals N 9 N, and take the course d e. 

 At the front of the lens they will again be refracted towards 

 the normals to that surface and take the course ef; at the 

 back of the lens, passing from a more refracting to a less re- 

 fracting medium, they will be bent from the normals N" and 

 take the course /#. If the retina be there, these parallel rays 

 will therefore be brought to a focus on it. In the resting 

 condition of the natural eye this is what happens to parallel 

 rays entering it : and, since distant objects send into the eye 

 rays which are practically parallel, such objects are seen dis- 

 tinctly without any effort, because all rays emanating from a 

 point of the object meet again in one point 011 the retina. 



Accommodation. Points on near objects send into the eye 

 diverging rays : these therefore would not come to a focus on 

 the retina but behind it, and would not be seen distinctly, 

 did not some change occur in the eye; since we can see them 



FIG. 149.-Diagram illustrating the surfaces at which light is refracted in the eye. 



quite plainly if we choose (unless they be very near indeed), 

 there must exist some means by which the eye is focussed or 

 accommodated for looking at objects at different distances. 

 That some change does occur one can, also, readily prove by 

 observing that we cannot see distinctly, at the same moment, 

 both near and distant objects. For example standing behind 



