GENERAL PRINCIPLE. 247 



in the same direction, is bent away from the perpendicular 

 and into the line o I. 



In almost all cases of light passing from one transparent 

 medium into another of a different internal constitution, 

 the ray is bent on this principle. The effect in most cases 

 seems to depend upon the difference of density iu the two 

 media, but not always. There is some mysterious quality 

 or condition of structure, not well understood, on which 

 the effect depends. The substance ordinarily employed 

 for refracting light is glass, and a glass formed with curved 

 surfaces to produce any of the various effects which may 

 be required is called a lens. Lenses are of various kinds, 

 according to the effect which it is desired to produce. A 

 double convex lens, for example, is convex on both sides. 

 Its effect upon parallel rays, according to the principle al- 

 ready explained, namely, that the light is bent toward the 

 side where the largest portion of the substance of the glass 

 comes nearest to it, or, in other words, toward the perpen- 

 dicular at the point where it enters, is to draw the rays 

 all inward, as we see is the case with a sun-glass, which is 

 an example of a double convex lens. On the other hand, 

 the same principle, in the case of a double concave lens, 

 which is the kind used for the eye-glasses of near-sighted 

 persons, will tend to spread the rays instead of drawing 

 them together; for while, in the convex lens, the thickness 

 of the glass increases toward the centre, in one that is con- 

 cave the thickness increases from the centre to the circum- 

 ference, and the light is drawn away toward the side 

 where the greatest thickness lies at the point at which it 

 enters. 



It is easy, in the same manner, for one who has the prin- 

 ciple of reflection in mind, as it has been explained in this 

 chapter, to see in what way light will be reflected from a 

 polished surface in any specified position or of any speci- 



