96 



THE HORSE. 



membrane and the pulp are perfectly transparent in the living animal. The 

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

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

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



On this expansion of nervous pulp, the rays of liglit from surrounding 

 objects, condensed by the lens and the humours, fall, and producing a 

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

 their existence and presence. 



Lii'ht consists of particles, which, proceeding from the sun or other 

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

 them, and entering 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 lines. It passes through some 

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

 obstructed 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 called 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, a, 



shall fall on a smooth or level piece of glass, in the direction a h, and that 

 course, if it were not acted upon by the glass, it would pursue. But ex- 

 perience 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 b, and the 



