236 optics. 



The cause of this refraction, or altering of the 

 direction of the rays, may be owing to the superior 

 attraction of the denser medium, which acts only 

 at a very small distance; on the contrary, when 

 light passes from a denser to a rarer medium, 

 it is most strongly attracted by that which it 

 leaves. 



It is owing to the refraction of light by the 

 atmosphere, that we see the sun and stars before 

 they are actually above the horizon, and also after 

 they are below it. Let ABC (Plate 25. fig. 7.) 

 represent a portion of the earth's surface, and 

 D E F the atmosphere surrounding it, G the place 

 of the sun, and B E the horizon of the point B. 

 A ray, G E, from the sun while still below the 

 horizon, will be refracted upon its coming into the 

 atmosphere, so as to arrive at the point B ; and, 

 therefore, to an eye placed at B, the sun will ap- 

 pear at H on the horizon. And for the same 

 reason, the image of the sun will be seen after he 

 is actually set. Astronomers, in their calculations 

 of the places of the heavenly bodies, are obliged to 

 make allowances for this refraction. 



When a ray of light, CDEF, (Plate 25. fig. 8.) 

 passes through a piece of glass, the sides of which 

 are parallel, as A B, it suffers two refractions, 

 which destroy each, and the ray moves on in the 

 same direction as at first. It is first refracted in the 

 direction D F, and on leaving the glass again, into 

 the direction E F, which is parallel to C D. 



If parallel rays, a b (Plate 12. fig. 5.), fall upon 

 a plano-convex lens, c d, they will be so refracted, 

 as to unite in a point, c, behind it ; and this point 

 is called the principal [focus, or the^cz^ of parallel 

 rays; the distance of which from the middle of 

 the glass is called the focal distance, which is equal 



