SEEING. 129 



sensible membrane, placed as a sort of guard to the delicate 

 organs "within, to protect them from any sudden changes of 

 light. When the light is strong, the fibres of the iris dilate 

 towards the centre, arid the pupil consequently becomes 

 smaller, so that fewer rays are admitted. When the light is 

 weak, the pupil becomes larger, and admits a larger number 

 of rays. 



' The humors of the eye are three in number, the aqueous, 

 the crystalline, and the vitreous. The crystalline humor is 

 a double convex lens, situated a little way behind the iris. 

 The space which intervenes between this lens and the cornea 

 is filled up by the aqueous humor. This is nearly of the 

 consistence of water, and surrounds the iris entirely, permit- 

 ting its light and delicate fibres to float freely in it. The 

 vitreous humor occupies the remaining back part of the eye. 

 It is contained in a great number of thin, niembranaceous, 

 and transparent cells, which, when punctured, pour out a fluid 

 of the consistence of the white of an egg. The rays of light, 

 in passing through the humors, are refracted, become con 

 centrated, and produce an image upon the retina. This is a 

 membrane formed by the optic nerve, which enters from the 

 brain, is spread out on the inner surface of the back part of 

 the eye, and receives the impressions made by the rays of 

 light, after they have been concentrated by the humors. This 

 impression is transmitted to the brain, and is the immediate 

 cause of vision.' 



By this curious apparatus, all the phenomena of vision are 

 conveyed to the mind. But before we enter upon the man- 

 ner in which the different parts of the eye concur in trans- 

 mitting the rays of light and the images of objects to the re- 

 tina, it will be necessary to give some general ideas concern- 

 ing the nature of light, which is the universal medium 

 of vision. 



Light is supposed to consist of innumerable particles of 

 matter, which proceed in direct lines from every part of tu- 

 rn mous bodies, and constitute rays. The motion of light, 

 though not instantaneous, is inconceivably swift. It has been 

 discovered by philosophers, that rays of light coming from 

 the sun reach this earth in eight minutes. Now the distance 

 of the earth from the sun is so immense, that, supposing a 

 cannon-ball to move at the rate of five hundred feet in a 

 second, it could not come from the sun to the earth in less 

 than twenty-five years. At this rate, the velocity of light will 

 be above ten million times greater than that of a cannon-ball. 



