POLARISED LIGHT. 105 



2d. That the beam o differs from ^ e in nothing excepting 

 that the former has the same properties at the sides a b' that the 

 latter has at the sides c' and d'; or in general, that the diameter of the 

 beam, at the extremities of which the beam has similar properties, 

 are at right angles to each other, as a h' and c' and dl for example. 



These two beams, o, E e, are therefore said to be polarised, or to 

 be beams of polarised light, because they have sides or poks of different 

 properties and planes passing through the lines ah, cd; or a'V, c d', 

 are said to be the planes oi polarisation of each beam, because they have 

 the same property, and one which no other plane passing through 

 the beam possesses. 



Now it is a curious fact, that if we cause the two polarised 

 beams o, E e to ha united into one, or if we produce them by a thin 

 plate of Iceland spar, which is not capable of separating them, we obtain 

 a beam which has exactly the same properties as the beam ah c d oi 

 common light. Hence we infer that a beam of common light, ahcd, 

 consists of two beams of polarised light, whose plane of polarisation, 

 or whose diameters of similar properties are at right angles to one 

 another. If o is laid above E e, it will produce a figure like ahcd; 

 and we shall therefore represent common light by such a figure. If we 

 were to place o above E e, so that the planes of polarisation al b' and 

 d d' coincide, then we should have a beam of polarised light twice as 

 luminous as either o or E e, and possessing exactly the same pro- 

 perties ; for the lines of similar property in the one beam coincide with 

 the lines of similar property in the other. Hence it follows that there 

 are three ways of converting a beam of common light, abed, into a 

 beam or beams of polarised light. 



1st. We may separate the beam of common light, ahcd, into its 

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

 polarisation, abed, 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 reflecting crystal, which was first discovered 

 to exist in a transparent mineral substance called Iceland spar, cal- 

 careous spar, or carbonate of lirnie. This substance is admirably 

 adapted for exhibiting 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 

 * Brewster's Optics. 



