LECT. XX. LONG-SIGHTEDNESS. 375 



brane, will be very slightly modified, in order that the im- 

 pression may not be sensibly altered, and that the percep- 

 tion may preserve all its distinctness. 



That which we have said of an isolated luminous point, 

 is applicable to each of the points of the illuminated ob- 

 ject, placed before the eye ; and it is easy to understand 

 how this new theory of the path of the luminous rays 

 through the refracting media of this organ, accounts for 

 such an important fact, and one which seemed difficult to 

 explain, namely, that of the distinctness of vision at every 

 distance. 



Long-sightedness. Presbyta, or long-sighted persons, see 

 objects distinctly at the distance of two or three feet. In 

 their eyes the cornea is less convex than that in a perfect 

 eye ; and, in fact, this defect of sight comes on with old 

 age, and follows the general diminution of the secretions of 

 all the tissues. By this flattening of the cornea, the focal 

 interval of the rays, which emanate from the point of dis- 

 tinct vision of the sound eye, is thrown behind the retina, 

 and, therefore, long-sighted persons are under the necessity 

 of increasing the distance of an object, in order that its 

 image may be formed on this membrane. Those who 

 suffer from this defect, usually have the pupil but little 

 dilated, as if a continual effort was made to use the cen- 

 tre only of the crystalline lens, namely, the most refract- 

 ing portion. To correct this defect, they are obliged to 

 employ convex lenses, which diminish the divergence of 

 the rays before they enter the eye. By this means the rays, 

 emanating from an object placed at the point of ordinary 

 vision, are bent by these lenses and brought into the di- 

 rection they would have if the object were situated at the 

 distance at which a long-sighted person saw distinctly. 



Short-sightedness. The other defect of the eye, myopia, 

 or short-sightedness, arises from an opposite cause ; namely, 



