342 WELLS'S NATURAL PHILOSOPHY. 



peculiar conditions are said to possess poles, because in some positions they 

 can be reflected and in others thej can not, and these positions are at right 

 angles to one another. 



^ . , ^ ,. 702, The phenomenon of polarized light was discovered in 



Explain the dis- ,,,7 . ^ ,. r^ ■ ^ 



coveryandphe- 1808, by Malus, a young engmeer oiScer oi Fans. On ono 

 iioniena of po- occasion, as he was viewing through a double refracting 

 prism of Iceland spar the light of the sun reflected from a glass 

 window in one of the French palaces, ho observed some very peculiar effects. 

 The window accidentally stood open like a doc* on its hinges at an angle of 

 54°, and Malus noticed that the light reflected from this angle was entirely 

 altered in its character. 



Tliis alteration in the character of the light reflected from the glass window, 

 which was thus first observed by Malus, may be made clear by the following 

 experiment : — Suppose we have a cylinder with a mirror at one end of it. If 

 we point this to the sun, and receive the image on a distant screen, we may 

 turn the cyUnder round on its axis, and the reflected ray will be found to revolve 

 constantly with it. But if now, instead of receiving the ray direct from the sun, 

 we allow a beam reflected from a glass plate, at an angle of about 54°, to fall 

 upon the mirror, and then be reflected on the screen, it will be found that the 

 point of light will not have the same properties as that previously examined ; 

 it will bo altered in its degree of intensity as the cylinder turns round ; will 

 have points where it is very bright, and others where it <rill entirely disap- 

 pear. It is thus proved that light reflected from glass at an angle of about 

 54°, has undergone some pecuhar modification, or, as it has been termed, 

 has become polarized. 



Certain minerals, especially those called "tourmalines," have the prop- 

 erty of polarizing a ray of Ught transmitted through them. 



PiQ. 278. If a ray of light be caused to pass through 



a thin plate of tourmaUne, as c d, Fig. 278, 

 in the direction of the line a b, and be re- 

 ceived upon a second plate, e f, placed 

 symmetrically with the first, it passes 

 through both without difficulty; but if tho 

 second plate be turned a quarter round, aa 

 in the direction g h, the light is totally cut oS". 



noTTisthepoi- According to the undulatory theory, the dif- 

 arization of fgrence between common and polarized light 



l!;;ht explain- ■•■ ^ ° 



«d? may be explained by supposing that in com- 



mon light the vibrations of the ether which produce it 

 take place in every possible direction, transverse to the 

 path of the ray ; but in polarized light they take place 

 in only one direction, or are all in ono plane. 



Thus, in the passage of a ray of light through the plate of tourmaline, 

 only ono set of vibrations is transmitted, while tho others are absorbed 



