SENSE ORGANS. 



103 



the sclerotic and choroid, and then spreads as a thin 

 translucent membrane nearly, but not quite, as far for- 

 ward as the choroid. 



Contents. — The hollow globe thus described is filled 

 with materials as clear as finest glass. These are the 

 humors or lenses of the eye. They are three in num- 

 ber. The crystalline, or lens proper, is a clear, glassy, 

 double convex lens, one third of an inch in diameter 

 and one sixth of an inch in thickness, and somewhat firm 

 and elastic to the touch. It is placed just behind the iris 

 and in light contact with it. It is invested with a trans- 

 parent membrane, the capsule, which continues from its 

 margin outward as a curtain to attach itself all around 

 to the sclerotic *a little behind the iris, and thus serves 

 to hold the lens in its place. The lens and the lens cur- 

 tain divides the interior of the eye into two unequal 

 parts. The smaller anterior part is filled with the aque- 

 ous humor, the larger posterior part with the vitreous 

 humor. The aqueous humor is as liquid as water, the 

 vitreous humor about the consistence of soft jelly. All 

 of these may be regarded as lenses. The crystalline, 

 with its double convexity, is the lens par excellence ; the 

 aqueous, with its corneal surface, is also a powerful con- 

 vex lens; while the vitreous, on its anterior surface, is a 

 concave lens. 



As already said, all of these are normally clear. 

 Opacity of the crystalline, which often comes with age, 

 constitutes what is called cataract, and produces blind- 

 ness. 



FORMATION OF THE IMAGE. 



Now the whole of what has been described is an 

 elaborate instrument to form an image on the sensitive 

 retina. If we ask, "Why an image?" the answer is, 

 Without an image we would perceive light, but not an ob- 



