THE EYE, AND THE PHENOMENA OF VISION. 353 



increasing the convergence of rays of light passing through them, bring them 

 sooner to a focus in tlie eye, and thus produce the image upon the right point 

 of the retina.* 



Most persons of advanced age are troubled with long-sightedness, and are 

 obliged to use spectacles. The reason of tliis is, that as the physical organi- 

 zation of the body becomes enfeebled, the humors of the eye dry up, or 

 are absorbed, and in consequence of this, the cornea and crystalline lens 

 shrink and become flattened. 



Beside these defects of the eye, a person may have the sense of vision 

 impaired or destroyed by an injury or disease of the optic nerve, or by a dimi- 

 nution of the transparency of the crystalline lens ; the first of these cases is 

 called amaurosis, and is incurable — the second, which is called cataract, may 

 be cured. 



. ., . The images formed by the rays of light upon the retina are 



on the retina inverted. It may, therefore, be asked why all visible objects 

 whr do^we'not *^° °°* appear upside down? The explanation of this curious 

 see' them up- point, which has formed the subject of much dispute, appears 

 °^^' to be this: an object appears to be inverted only as it is com- 



pared with some other objects which are erect If all objects hold the same 

 relative position, none can be properly said to be inverted. Now, since all 

 the images produced upon the retina liold, with relation to each other, the 

 same position, none are inverted ^-ith respect to others ; and as such images 

 alone can be the object of vision, no one object of vision can be inverted with 

 respect to any other object of vision ; and, consequently, all being seen in the 

 same position, that position is called the erect position. 



. , 710. The optic axis of the eye is a line 



What is the t i i i 11 n 



optic axis of drawn perpendicularly through the center of 



the cornea, and center of the eye-ball. 

 ^s^dTwe^iirt "^^^^ reason why with two eyes we do not see 

 potnt'ofar^ double is, because the axis of both eyes is 

 ject double? turned to one point, and therefore the same 

 impression is made on the retina of each eye. 



The law of vision for visible objects is entirely different from that for points. 

 A visible object can not, in all its parts, be seen single at the same instant of 

 _ time, but the two eyes converge their axes to the near and the remote parts of 

 it in succession, and thus give an idea of the different distances of its parts. 

 Aaj defect which will prevent the two eyes from moving together conjointly, 

 and from converging their optic axes upon every point of an object in succes- 

 sion, will be fatal to distinct vision. 



• Birds of prey are enabled to adjus', their eyes bo as to see objects at a great distance, 

 and again those which are very near. The first is accomplished by means of a muscle in 

 the eye, which permits them to flatten the cornea by drawin;^ back the crystalline lens; 

 and to enable them to perceive distinctly very near objects, their ryes are furnished with 

 a tie.xible bony rim, by which the cornea is thrown forward at will, and the eye thus ren- 

 dered near-sighted. 



