THE CROOKED COURSES OF LIGHT, 



35 



in form and size to the real objects in front of them. Fig. 2 repre- 

 sents the formation of an image of a candle in a common looking-glass. 

 The reflection is shown as limited to the pencil of rays emitted by the 

 highest point of the flame. The reflected rays which enter the eye 

 are seen to be divergent like the incident rays, so that if they were 

 produced backward they would meet at a point forming the image at 

 the top of the flame. As all surfaces are made up of points, and each 

 point of the object is reflected in the same manner, it is clear that the 

 imao-e formed by a plane mirror must symmetrically represent the 

 object. 



Fig. 4. Fio. 5. 



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^. N-^^M ~l^l--^ 



Skell's Law op Refraction. 



But light-rays may be turned from their direct course in another 

 way. When a beam passes obliquely from one transparent medium 

 to another of different density, as from air to water or glass, its direc- 

 tion is chano-ed and it is said to be refracted. This is illustrated in a 

 very simple manner by Fig. 3, in w^hich a ray of the sun, entering 

 through an aperture in a dark room and received on the surface of 

 water in a glass vessel, is seen to be broken as it were at the surface 

 and bent downward. 



A familiar experiment illustrating the same principle is to put a 

 coin upon the bottom of an empty, opaque vessel, while the spectator 

 places himself so that it is just hidden by the vessel's edge. If water be 

 now poured into it, the bottom of the dish will appear to rise, and the 

 coin will come in sight. The pencil of rays thus undergoes a sudden 

 bend at the surface qf the water, and reaches the eye by a crooked 

 course, the eflfect of which is, that the spectator sees round or behind the 

 obstacle. Fig. 4 shows how an inclined stick, partially immersed in 

 water, presents a broken appearance. Transparent substances diffier 

 in this refracting power. Liquids exhibit it in a much higher degree 

 than gases, and, as a general rule, the denser of two substances mani- 

 fests the greater refracting effect. Hence it is common to speak of 

 the change in the ray as it passes from a denser into a rarer medium, 

 or the reverse. 



