352 



WELLS'S NATURAL PHILOSOPHY. 



How is the eye 

 enabled to see 

 objects dis- 

 tinctly at differ- 

 ent distances ? 



Pig. 285. 



As the rays of light proceeding from distant objects enter 

 the eye at diflerent angles, they will naturally tend to meet 

 at diflerent foci after refraction by the crystalline lens, and 

 thus form indistinct images. This is remedied by a power 

 which the eye possesses of adapting itself to the direction of 

 the light proceeding from various distances, so that in the healthy eye, rays 

 coming from near and distant objects are all equally converged to a focus on 

 the same point of the retina. How the eye effects this is not certainly known, 

 but it is supposed to be by increasing or diminishing the sphericity of tho 

 ca'ystalline lens and cornea. 



What is tho ^ person is said to be near-sighted when 

 efghTednersT" ^^^^ curvuture of the cornea and crystallino 

 lens is so great, that the rays of light which 

 form the image are brought to a focus before they reach 

 the retina, or the back part of the eye. The object, there- 

 fore, is not distinctly seen. 



Fig. 285 represents the manner 



in which the image is formed in 



the eye of a near-sighted person. 



Tho curvature of the cornea, s s, 



and of tho crystalline lens, c c, is 



so great that the image is formed 



at m m, in advance of the re- 

 tina. 



Short-sightedness is remedied cither by holding the objedt 

 nearer to the eye, or by the employment of spectacles tho 

 glasses of which are concave lenses. In both cases the rays 



proceediag from the object enter the eye with a greater degree of divergence, 



and therefore do not converge so soon to a focus. 



A person is said to be far-sighted when, on 

 account of a flattening of the cornea and the 

 crystalline lens, the rays of light do not con- 

 verge sufficiently to form a distinct image upon the retina. 



Fig. 286, represents the manner 

 in which the imago is formed in 

 the eye, when the cornea or crys- 

 talline lens is flattened. The per- 

 fect image would be produced at 

 m m, behind the retina, and, of 

 course, beyond the point necessary 

 to secure distinct vision. 

 Long-sightedness may be remedied by the employment of 

 spectacles, tho glasses of which are convex lenses. These, by 



Xlow is short 

 Eiglitedness 

 remedied J" 



What is the 

 cause of far- 

 Bightedness ? 



Fig. 286. 



How may long, 

 sightedness be 

 remedied ? 



