576 LENSES AND THEIR ACTION [Ca. XIV 



801. Refraction. By refraction is meant the change in 

 direction of a ray of light in passing from one transparent medium 

 into another. 



The amount of bending depends upon two conditions : 



(1) The greater the angle of incidence of the light, that is, the 

 farther from the perpendicular or normal that the light strikes the 

 surface, the greater will be the bending on entering the second 

 medium. And this increase is not simply with the increase of the 

 angle of incidence, but proportionally greater, that is, in accordance 

 with the law of sines (fig. 312). 



(2) The bending also depends upon the difference of density 

 of the two transparent media. If the difference is great, the 

 refraction will be great, and if the difference of density is small, the 

 refraction will be proportionally small. See also chromatic aberra- 

 tion (8 10, fig. 337). 



The phenomena of refraction were worked out with great 

 accuracy by Ptolemy in the first and beginning of the second cen- 

 tury A.D. ; but the precise mathematical expression for the law of 

 refraction was not found until about 1500 years later (Snell's and 

 Descartes' law of sines). This law of sines includes both elements 

 mentioned above, and is expressed thus : 



Sine of the angle made by the incident ray 



-- = index of retraction. 



Sine of the angle made by the refracted ray 



802. Lens. By making one or two bounding surfaces of a 

 transparent body curved, rays of light traversing the body are 

 made to converge or to diverge. Any transparent body having one 

 or both of its opposite faces curved is called a lens. The curved 

 surfaces are usually segments of spheres, as a spherical surface can 

 be ground and polished more accurately than can any other. 



803. Principal axis. The straight line passing through the 

 centers of the two spheres of which the surfaces of a lens are seg- 

 ments is called the principal axis. This axis is perpendicular to 

 both surfaces of the lens (fig. 314). 



804. Optic center. This is the point in a lens, or near it, 

 through which light rays pass without angular deviation, that is, 



