Human Physiology. 191 



camera obscura a dark chamber like the box used by the 

 photographers. The pupil is the round tube pointed toward 

 the sitter, but in the eye it grows larger or smaller with every 

 change of light. The lens of the photographer is shaped like 

 that of the eye, and moved backward and forward with a screw 

 to secure a focus, but the soul within adjusts the lens of the eye 

 from instant to instant to every object of vision. The rays of 

 light from the sitter form a picture on a plate of ground glass, 

 and then on the sensitive plate of the negative. In the eye 

 pictures are continually formed upon the retina, upside down as 

 in the camera, and of microscopic minuteness, but the nerve 

 spread out to form the retina carries the impression erect and 

 perfect to the mind. It has been imagined that the sense of 

 touch was needed to correct the defects of vision, but there are 

 no such defects. Chickens just out of the egg judge of dis- 

 tances perfectly. Ducklings hurry off to the nearest water. 

 The butterfly sees as well the first hour of his life as the last. 

 The whole mechanism is simply perfect, and of itself enough to 

 convince any reasonable being that it must be the work of an 

 Infinite Artist, a Mechanician all-wise and almighty. 



I have spoken of the muscles that move the eye in a former 

 chapter, one of which passes through a cartilaginous pulley to 

 change the direction of the moving force ; but the lovely curtain 

 that forms the pupil of the eye, the iris, is drawn back and for- 

 ward by a most complicate apparatus. The eye is washed 

 every moment and kept clean and bright by a gland above it 

 secreting a salt liquid and the involuntary opening and shutting 

 of the lids, while the superfluous liquid passes off by a tube into 

 the nose. Beautiful lashes shade the eye from glare and protect 

 it from dust, and comely eyebrows guard it against the sweat of 

 the brow. How beautiful as well as useful are brow and lashes 

 any one may see who chooses to cut them off and do without 

 ihem. 



How wonderful is vision, and how wonderfully adapted is 



