416 REFRACTION BY LENSES: FORMATION OF IMAGES. 



the curvature of the lens is more considerable. Thus a convex 

 lens has a long focus or a short focus (that is, brings rays to a 



Fig. 207. 



focus at a greater or less distance from itself) according as the 

 curvature of its surface is less or more considerable. 



531. The rays issuing from every point in an object, and 

 falling upon a convex lens, are brought to a focus on the 

 other side of the lens ; and thus a distinct image or picture 

 is formed upon any screen placed at the proper distance to 

 receive it (as is seen in a Camera Obscura or a Magic Lantern), 

 every point in that image being the representative of the 

 corresponding point in the object, but this image being 

 inverted. 



532. JSTow, the Eye, in its most perfect form such as it 

 possesses in Man and the higher animals is an optical 

 instrument of wonderful completeness, designed to form an 

 exact picture of surrounding objects upon the expanded sur- 

 face of the optic nerve, by which its impression is conveyed 

 to the brain. As it is in the most perfect form of this instru- 

 ment that we are best able to judge of the uses of its different 

 parts, it will be preferable to consider this in the first instance, 

 and then to advert to the less complete forms which we meet 

 with in the lower animals. 



533. The Eye of Man, like that of all Vertebrata, has a 

 nearly globular form. The walls of the sphere are composed 

 of three coats; whilst in its interior are found three humors 

 of a more or less fluid character. The outer coat, named the 

 Sclerotic (s s, fig. 208), is tough and fibrous, and is destined 

 to support and protect the delicate parts which it contains. 

 It does not cover the whole globe, however ; but gives place 

 in the front of the eye to a transparent lamina of cartilaginous 



