POLARISATION OF LIGHT. 



Fig. 20. 



the four images will be all equal, .is at H 5 

 farther on at I, they will become un- 

 equal ; and at 180 of revolution, they 

 will all coalesce into one bright image, 

 as at K. 



From these results, it follows, that in 

 the position of the two rhombs shown in 

 fig. 19, which are separated a little, and 

 where the planes of the principal sections 

 are parallel to each other, the pencil 

 D C, which was regularly or ordi- 

 narily refracted by the first rhomb M, 

 has not suffered double refraction by the 

 second rhomb N, but has only suffered 

 ordinary refraction in the line G H, and 

 emerges as a single pencil corresponding 

 to one of those at B,Jig- 2 ; while the pen- 

 cil C E, which was extraordinarily refract- 

 ed by the first rhomb M, is now only extra- 

 ordinarily refracted by the second rhomb 

 N, and emerges as a single pencil corre- 

 sponding- to the other at B, fig. 20. After 

 a rotation of 90, when the planes of the 

 principal sections are at right angles to 

 one another, two images are only seen as 

 at F : but in this case the ray D G, which 

 proceeds from the ordinary refraction, 

 suffers only the extraordinary refraction 

 in GH ; and the ray C E , which pro- 

 ceeds from the extraordinary refraction, 

 suffers only the ordinary refraction in 

 E F. In all other positions beside these, 

 in which the planes of the principal 

 sections of M and N are parallel and 

 at right angles to each other, each 

 of the rays C E, D G, are divided into 

 two, as shown at D, C, G, H, I,- E, 



fig- 20. 



The four images thus described may 

 be expressed in the following manner : 

 O the pencil refracted ordinarily by the 



first rhomb. 

 o the pencil refracted ordinarily by the 



second rhomb. 

 E the pencil refracted extraordinary 



by the first rhomb. 

 e the pencil refracted extraordinarily by 



the second rhomb. 



Then the pencils which actually emerge 

 at F, H will be thus expressed : . 

 Oo the pencil refracted ordinarily by 



both rhomboids. 

 O e the pencil refracted ordinarily by 



the first, and extraordinarily by the 



second. 

 E o the pencil refracted extraordinarily 



1 K 



by the first and ordinarily by the 



second. 

 E e the pencil refracted extraordinarily 



by both rhombs. 



Then, according to Malus, if we sup- 

 pose L to be the intensity of the light 

 incident at B, and a the angle formed 

 by the principal sections, and set aside 

 the consideration of the light lost by re- 

 flexion and absorption, we shall have 



O o = i L cos. s a = E e 



O e = | L sin. 2 a = E o, and 



CHAPTER V. 



Description of Apparatus for Experi- 

 ments on Polarised Light. 



Having thus described the various 

 ways by which common light may be 

 polarised, we must now describe the dif- 

 ferent kinds of apparatus which are 

 necessary for investigating the wonderful 

 phenomena which next demand our at- 

 tention. There are two different kinds 

 of apparatus one for polarising light, 

 and another for analysing polarised light. 



1. Single Reflecting Planes. Light 

 may be conveniently polarised by a 

 single plate of any transparent body 

 without double refraction, such as glass, 

 obsidian, ebony ; or by a single surface 

 of water, oil, treacle, any varnished body, 

 or any ordinaiy crystallised surface. But 

 in selecting any plate or surface, it should 

 be one which has a low dispersive and 

 refractive power; for it is only in this 

 case that the reflected light will be com- 

 pletely polarised. Glass of antimony, 

 oil of cassia, flint glass of high refractive 

 power, coloured or stained glasses, are 

 all unsuitable for this purpose. A plate of 

 thin well-annealed crown glass (if with 

 parallel surfaces, so much the better) 

 will answer for ordinary experiments. 



2. Reflecting Bundles of Glass Plates. 

 When a great deal of light is required, 

 which is frequently the case, especially 

 when we use the microscope for examin- 

 ing imperfectly transparent bodies, from 

 one to sixteen plates of the clearest 

 and thinnest annealed glass should be 

 placed in a frame, having their surfaces 

 well washed and cleaned with fresh 

 chamois leather. Their edges must 

 then be covered with some cement or 



