144 THE MICROSCOPE. 



of common light by a circle with two diameters a b, c d, at 

 right angles to each other. 



If we now allow the same beam of light to fall upon a 

 rhomb of Iceland spar, and examine the two circular 

 beams, o E e, formed by double refraction, we shall find, 

 1st, that the beams o E e have different properties on 

 different sides, so that each of them differs in this respect 

 from the beam of common light. 



2d. That the beam o differs from E e in nothing ex- 

 cepting 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 diameters of the beam, at the extremities 

 of which the beam has similar properties, are at right 

 angles to each other, as a b' and c and d' 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 poles of different properties and planes passing 

 through the lines a b, c d ; or a' 6', c' d', are said to be the 

 planes of 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 be 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 a b c d of common 

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

 c dj 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 abed] and we shall 

 therefore represent polarised light by such figures. If we 

 were to place o above E e, so that the planes of polarisa- 

 tion d b' and c' 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 properties; 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, a b 

 c d, into a beam or beams of polarised light. 



1st. We may separate the beam of common light, a b c d, 



