30 LIGHT. 



tant ; and were it not for the judgment, the nearer might 

 be supposed to be a man, and the more distant a little boy. 



As only a very small part of an object can be seen dis- 

 tinctly at once, the eye is forced to turn itself successively 

 to the different parts it is desirous of viewing. When an 

 object is seen with both eyes, the axes of both eyes are 

 directed to the object, and meet there j and the optic 

 nerves are so formed, that the correspondent parts of 

 each eye lead to the same spot in the brain, and produce 

 but one sensation. If the axes of both eyes be not di- 

 rected to the same object, the object will appear double, 

 as the pictures do not fall on similar parts of the retina 

 in both eyes. This may be perceived by holding the 

 finger before any object, a lighted candle for instance, 

 and by looking steadfastly at the object there will seem to 

 be two fingers, and by looking at the finger the object 

 will appear double. The smallest visual angle is about 

 half a minute of a degree, and at a medium not less than 

 two minutes. To most eyes, the nearest distance for dis- 

 tinct vision is seven or eight inches. 



The external shape of the eye is found to affect the 

 sight. The eyes of some persons are naturally too con- 

 vex; some acquire too great a convexity of the eye by 

 close reading, which alters the shape of the crystalline 

 humour j the image then becomes formed too soon, from 

 the rays meeting in a focus before they reach the retina, 

 unless the object be brought near the eye, in which case 

 the image will be cast farther back. Shortsighted per- 

 sons, therefore, use concave glasses to view objects at a dis- 

 tance, which diverge or spread the rays, and render the 

 vision distinct. The eyes of persons in the wane of life 

 generally become less and less convex, the image will 

 not then be formed soon enough on the retina ; convex 

 glasses will therefore be used, by which means the rays 

 of light are converged, and the image is clearly delineated. 



According to Sir I. Newton, Light is not a body of a 

 homogeneous nature, but consists of rays of different 

 kinds or colours,* each of which, in passing from one 

 I 



* Colour is considered a property inherent in light, by which it ex- 

 cites different vibrations in the optic nerve, which being conveyed to 

 the brain affect the mind with different sensations. According to Sir 



