m 



THE HORSE. 



brane and the pulp are perfectly transparent in the living animal. The pupil 

 appears to be black, because in the clay time it imperfectly reflects the 

 colour of the choroid coat beneath ; in the dusk it is greenish, because, the 

 glare of day being removed, the actual green of the pahit appears. 



On this expansion of nervous palp, the rays of light from surround- 

 ing objects, condensed by the lens and the humors, fall, and producing a 

 certain image corresponding with these objects, the animal is conscious of 

 their existence and presence. 



Light consists of particles, which, proceeding from the sun or other lu- 

 minous bodies, fall on different objects, and being again reflected from 

 them, and enterhig the eye, render these objects visible. If we are in a 

 dark room, which we know to be filled with furniture, we see it not, and 

 were it not for our previous knowledge of it, or did we not touch it, we 

 should not be conscious of its existence ; but if a candle be brought into 

 the room, or if one of the shutters be opened, the light from the candle, 

 or that admitted through the window, falls upon the different articles of 

 furniture, and a portion of it being reflected from them, and reflected in 

 every direction, some of the light enters the pupil of the eye, and we 

 see the objects around us. 



It proceeds from these objects to us in straight lines, and except it were 

 impeded, or driven, or drawn out of its course by some body, it would 

 continue to travel on for ever in straight Unes. It passes through some 

 bodies with perfect ease, as glass, and crystal, and water, but it is ob- 

 structed in its passage by others, as metals and wood. These substances 

 through which it readily passes are said to be transparent; those by which 

 its course is arrested are cafled opaque. It has an attraction for all bodies, 

 stronger for some than for others. By the opaque body the greater part of 

 it is absorbed, and although it passes through the transparent body, it 

 feels and is affected by the attraction of that body. It is bent out of its 

 way although not detained. This is called the refraction of light ; and it 

 is on the knowledge of this simple fact that all our optical instruments are 

 constructed, and that we are enabled to explain the wonderful construction 

 of the eye. 



This little figure will make it sufficiently evident. A ray of light, «, shall 



fall on a'smooth or level piece of glass, in the direction a b, and that course, 

 if it were not acted upon by the glass, it would pursue. But experience 

 teaches us that it does not. It no sooner enters the glass, than it is bent 

 out of its original course, and takes the direction d. It had been acted 

 upon by two forces, the first impulse in the direction a 6, and the attraction 



