ANIMAL MECHANISM. 125 



far back as to place it on the retina. Old age, the de- 

 struction of the first eye, eventually restores the near- 

 sighted, by the gradual flattening of the cornea, till at 

 threescore and ten, such persons can see clearly and 

 distinctly without artificial aid. Many near-sighted 

 people totally ruin the organ by prematurely wearing 

 glasses, as a focus is established which neither glasses can 

 keep pace with in age, nor age thoroughly overcome. 

 FIG. 13 



Explanation of Figure 13. 

 The effect of this glass being exactly the reverse of the convex, 

 it causes the rays to fall upon the surface of the eye, so far di- 

 verging from the perpendicular line, as to correct the too great con- 

 vergence, caused by the convexity of the humors. When a near- 

 sighted person has brought the object near enough to the eye to see 

 it distinctly, he sees more minutely and consequently more clearly ; 

 because he sees the object, says Mr Bell, larger, and as a person 

 with a common eye does, when assisted with a magnifying glass. 

 A near-sighted person sees distant objects indistinctly , and, as the 

 eye, in consequence, rests, says the same observing writer, with 

 less accuracy upon surrounding objects, the piercing look of the 

 eye is very much diminished ; and it has, moreover, a dulness and 

 heaviness of aspect. Again, the near-sighted person knits his eye- 

 brows, and half closes the eye-lids ; this he does unconsciously, to 

 change the direction of the rays, and to correct the inaccuracy of 

 the image. Near-sighted people have but little expression ; the 

 countenance loses all its majesty, by habitually wearing glasses. 



THE IMAGE OF AN OBJECT IN THE EYE, IS INVERTED. 



Rays of light going from the upper and lower points of 

 an object, are refracted towards the perpendicular ; that is, 

 bent out of the course which they have a tendency to 

 run, by the crystalline lens behind, where they unite in 

 a point, and, then crossing, diverge again. Here then, 

 the image is bottom upward, as will be noticed in the 

 preceding diagrams by the arrow, and its image on the 

 retina. Decussation is indispensable to the vision of 

 things. An object could not be represented on a point ; 



VOL. I. NO. V. 11* 



