1814.] made during the Year 1313. 5 



round, while the first remains stationary, each of the pencils begins 

 to be separated into two; and when the eighth part of a revolution 

 is completed, the whole of each of the pencils is divided into two 

 portions. When the fourth part of a revolution is completed, tlie 

 pencil refracted in the ordinary way by the first crystal will be 

 refracted in the extraordinary way only by the second, and the 

 pencil refracted in the extraordinary way by the first will be re- 

 fracted in the ordinary way only by the second : so that the four 

 pencils will be again reduced to two. At the end of f, a, and i 

 of a revolution, the same phenomena will be exhibited as at the 

 end of -1- of a revolution. At the end of i and f of a revolution, 

 the same phenomena will be seen as at the first position of the 

 crystals and at the end of f of a revolution. If '^e look at a 

 luminous object through the two rhomboids, we shall at the coni- 

 raencement of the revolution see only two images, viz. one of the 

 least and one of the greatest refracted images. At the end of \ of 

 a revolution four images will be seen ; and so on. 



It is obvious that the light which forms these images has suffered 

 some new modification, or acquired some new property, which 

 prevented it in particular parts of a revolution from penetrating the 

 second rhuml)oid. This property has been called polar izut ion; and 

 light is said to be polarized by passing through a rhomboid of cal- 

 careous spar, or any other doubly refracted crystals. 



Some years ago Malus announced the discovery of a new pro- 

 perty of reflected light. He found that when light is reflected at a 

 particular angle from all transparent bodies, whether solid or fluid, 

 it has acquired by reflection that remarkable property oi polarization 

 which had hiiherto been regarded as the ed'cct only of double 

 refraction. If the light of a taper reflected from the surface of 

 water at an angle of 52° 45', be viewed through a rhomboid of 

 Iceland crystal, which can be turned round the axis of vision, two 

 images of the taper will be distinctly seen at one position of the 

 crystal. At the end of a. of a revolution one of the images will 

 vanish ; and it will reaj)pear at the end of f of a revolution. The 

 other itnage will vanish at the end of f of a revolution ; and will 

 reappear at the end of a : and the same phenomena will be repeated 

 in the other two quadrants of its circular motion The light 

 reflected from the water, then, has been evidently polarized, or 

 has received the same character as if it had been transmitted 

 through a doubly refracting crystal. 



The angle of incidence at which this modification is superinduced 

 upon reflected light increases in general with the refractive power 

 of the transparent body : and when the angle of incidence is 

 greater or lc>s than this particular angle, the light suflcrs only a 

 partial m )diticatl(jn, in the same manner as when two rhomboids of 

 Iceland sp ir an- not placed either in a similar or in a transverse 

 position. 



Mains found that light reflected from opaque bodies, such as 

 black marble, ebony, &c. is aUo polarized : and shortly before his 



