NATURAL HISTORY. 43 



" Time wasted is existence, used is life, 

 And bare existence, man, to live ordained, 

 Wrings and oppresses with enormous weight." YOTJNO. 



121. Why does the pupil of the eye contract and dilate ? 



Because when it is necessary to exclude excess of light, the pupil 

 through which the light enters may gradually or wholly exclude it 

 by contraction ; and when a large amount of light is desired, the 

 dilation of the pupil in a similar degree ensures the admission of 

 a greater number of rays. 



122. The chamber of the eye is a camera-obscura, which, when the light is too 

 small, can enlarge its opening ; when too strong, can again contract it ; and that 

 without any other assistance than of its own exquisite machinery. 



123. Why is a person unable to discern objects when passing from, 

 a strongly-illuminated room into one comparatively dark, or into 

 the open air at night ? 



Because the contraction of the pupil, which was adapted to the 

 strong light to which it had been previously exposed, admits so little 

 light to the retina that no sensation is produced. The pupil, how- 

 ever, after a while dilates, and, admitting more light, objects are 

 perceived which were before invisible. 



124. Why does the eye of a person suffer inconvenience and 

 pain in passing from a dark room into a light one ? 



Because, while the observer remains in the dark or less illu- 

 minated room, the pupil is dilated to that degree so as to admit 

 into the eye as great a quantity of light as the structure of the organ 

 allows of. When he passes suddenly into the strongly-illuminated 

 room the flood of light arriving through the widely dilated pupil 

 acts with such violence upon the retina as to produce pain, which 

 necessarily calls for the relief and protection of the organ. The 

 iris, then, by an action peculiar to it, contracts the dimension of the 

 pupil so as to admit proportionally less light, and the eye is 

 gradually opened with impunity. 



125. Why is tlie pupil of the eye so called? 



Because if we look into the eye of another we shall perceive a 

 little image of our own face, like a very minute child or pupil 

 hence the name. 



