130 SENSES. 



The rays of light, though they proceed in direct lines from 

 luminous bodies, are refracted, or bent out of their course, in 

 passing through different mediums, as the air, glass, and 

 every transparent substance ; but when they fall upon opaque 

 bodies, they are reflected. Rays proceeding from any object, 

 and passing through a convex glass, or lens, are refracted 

 and collected into a point, or small space, at a certain dis- 

 tance from the glass, which is called the focus of that lens. 



The different humors of the eye, and the crystalline lens, 

 are all denser than air or water ; of course, their power of 

 refracting the rays of light is likewise greater. The. rays, 

 proceeding from every point of an object, enter the pupil ; 

 and the refraction of the different parts of the eye, which act 

 as a lens, necessarily makes them cross each other in their 

 passage to the retina. After crossing, they diverge till they 

 are stopped by the retina, whgre they form an inverted pic- 

 ture. /The upper part of the object is painted on the lower 

 part of the retina, and the right side upon the left, &c. The 

 celebrated Kepler first discovered, that distinct, but inverted 

 pictures of every object we behold, are painted on the retina 

 by the rays of light proceeding from visible objects. This 

 discovery naturally led Kepler, as well as many other philos- 

 ophers since his time, to inquire how we should see objects 

 erect from inverted images on the retina. 



Many ingenious theories have been invented, in order to 

 explain this seemingly difficult question. To give even a 

 cursory view of them would not only be tedious, but in a great 

 measure useless. We shall therefore only remark, that their 

 authors have uniformly assumed the principle, without prov 

 ing it, that because the pictures are inverted on the retina, 

 the mind ought also to perceive them in the same position. 

 But this does by no means follow, and we can only resolve it 

 into this, that animals see objects in their real position, 

 although their images are inverted by a law of nature. It is 

 certain, that, unless distinct images are painted on the retina, 

 objects cannot be clearly perceived. If, from too little light, 

 remoteness, or any other cause, a picture is indistinctly painted 

 on the retina, an obscure or indistinct idea of the object is 

 conveyed to the mind. The picture on the re-tina, there- 

 fore, is so far the cause of vision, that unless this picture be 

 clear and well denned, our ideas of the figure, color, and 

 other qualities of any object presented to the eye, will be ob- 

 scure and imperfect. The retina of the eye resembles a can- 

 vass on which objects are painted. The colors of these pic- 



