POLARISED LIGHT. 135 



component parts o and E e. 2d. We may turn round 

 the planes of polarisation, ah c d, till they coincide or are 

 parallel to each other. 3d. We may absorb or stop one of 

 the beams, and leave the other, which will consequently 

 be in a state of polarisation," ^ 



The first of these methods of producing polarised light 

 is that in which we employ a doubly refracting crystal, 

 and was first discovered to exist in a transparent mineral 

 substance called Iceland spar, calcareous spar, or carbonate 

 of lime. This substance is admirably adapted for exhibit- 

 ing this phenomenon, and is the one generally used by 

 microscopists. Iceland spar is composed of fifty-six parts 

 of lime and forty-four parts of carbonic acid ; it is found 

 in various shapes in almost all 

 countries; but whether found in 

 crystals or in masses, we can always 

 cleave it or split it into shapes re- 

 presented by fig. 81, which is called 

 a rhomb of Iceland spar, a solid 

 bounded by six equal and similar 

 rhomboidal surfaces, whose sides 

 are parallel, and whose angles ha c^ Fig gi 



a c d, are 101° 65' and 78° f/. The 



line a x, called the axis of the rhomh, or of the crystal, is 

 equally inclined to each of the six faces at an angle of 

 45° 23.' It is very transparent, and generally colourless. Its 

 natural faces when it is split are commonly even and per- 

 fectly polished; but when they are not so, we may, by a 

 new clevage, replace the imperfect face by a better one, 

 or we may grind and polish an imperfect face. 



It is found that in all bodies where there seems to be 

 an irregularity of structure, as salts, crystallised minerals, 

 &c., on light passing through them, it is divided into two 

 distinct pencils. If we take a crystal of Iceland spar, and 

 look at a black line or dot on a sheet of paper, there will 

 appear to be two lines or dots; and on turning the spar 

 round, these objects will seem to turn round also; and 

 twice in the revolution they will fall upon each other, 

 which occurs when the two positions of the spar are exactly 

 opposite, that is, when turned one-half from the j)osition 



(]) Brewster's "Optics." 



