THE REASON WHY. 233 



** Keep me as the apple of thine eye ; hide me under the shadow of thy 

 wings." PSALM xvn. 



969. Why is the pupil of the eye larger sometimes than 

 it others ? 



Because the iris, a ring of extremely fine muscles which surround 

 the pupil, contracts when too much light falls upon the retina, and 

 dilates when the light is feeble. It therefore enlarges or 

 diminishes the size of the pupil to regulate the admission of 

 light. 



Fig. 62. SECTION OF THE EYE SEEN FROJI BEHIND 



A. The pupil of the eye through which the li'j;ht enters. 



B. The iris, which dilates or contracts, and thereby increases or lessens the 

 size of the pupil. 



C C. The three coats of the eye, called the sclerotic, choroid, and retina. 



D. The ciliary processes, or hair-like muso'es, which have a slight vibratory 

 motion which they impart to the fluids of the eye. 



E. The dark coat of the choroid, the coat forming the retina removed. 



970. Why have we two eyes ? 



Because the field of vision is thereby much extended; the 

 intensity of sight is also increased, the impressions upon the brain 

 being clearer and better defined, just as in a stereoscope the effect of 

 vision is heightened by a double picture ; the sense of sight being 

 more constantly exercised than any other sense during our waking 

 moments, one eye is frequently called upon to give rest to the 

 other ; and the important faculty of vision, being endangered by 

 the necessary exposure of some parts of the eye, and the equally 



