1 THE 



DOUBLE REFRACTION AND POLARISATION 



OF 



LIGHT. 



PART I. 

 OX THE DOUBLE REFRACTION OF LIGHT. 



INTRODUCTION. 



Simple Refraction Double Refraction 

 in Iceland Spar described Ordinary 

 and Extraordinary Ray Double Re- 

 fraction possessed by various Mineral, 

 Animal, and Vegetable Bodies. 



IN explaining the various optical phe- 

 nomena which arise from the transmis- 

 sion of light through transparent bodies, 

 it is always supposed that these bodies 

 are perfectly homogeneous, and have the 

 samp temperature and density through- 

 out their mass. 



In such substances as pure water, and 

 well annealed glass, the minutest point, 

 or the finest line, will appear single when 

 seen in any direction or through any 

 thickness, provided that the mass is 

 bounded by parallel faces. If the 

 water or the glass have a prismatic 

 shape, a luminous point or a luminous 

 line will still appear single through the 

 prism, if the light, which issues from 

 the point or the~line, is homogeneous or 

 simple. But if the light is white, there 

 will be a red, orange, yellow, green 9 

 blue, indigo, and violet image. In like 

 manner, if the mass of water or glass, 

 when bounded by parallel surfaces, is 

 heated unequally so as to produce strata 

 of different density, and consequently of 

 different refractive power, then, if the 

 luminous point or line is seen in the 

 direction of the strata, a double, and 

 sometimes a triple image of it will ap- 

 pear, as in the phenomena of the mirage 

 or unequal refraction. 



In all these cases, however, whether 

 the object is seen double, treble, or 

 quadruple, the phenomena are those of 

 single or simple refraction, because 

 they are all produced by the same at- 

 tractive force, varying only in the degree 

 of its intensity ; and are all regulated by 

 the simple law of the sines discovered by 

 Snellius. The existence of more than 



one image is not a proof of the exist- 

 ence of more than one force, unless the 

 substance is perfectly homogeneous, of 

 equal density throughout, and bounded 

 by parallel faces. 



* A substance called Iceland spar, cal- 

 careous spar, or carbonate of lime, has 

 been long known to mineralogists. It 

 is found" in masses often larger than 

 one's head. It is composed of 56 parts 

 of lime, and 44 of carbonic acid, and 

 has a specific gravity of 2.714. It is 

 perfectly transparent and colourless, and 

 is susceptible of taking a fine polish. 

 When broken, it occurs in pieces of the 

 form shown in fig. 1., which is a solid 



_. 1. 



called a rhomb or rhomboid, bounded 

 by parallel faces which are inclined to 

 each other, at an angle of 105 5'. 

 These natural faces are often even and 

 perfectly polished ; and as the mineral 

 cleaves or splits parallel to any of its 

 six faces, it is easy to replace an im- 

 perfect face by a new one- 

 Having obtained a rhomb of Iceland 

 spar with smooth faces, place it, as 

 shown in Jig* 2, above a sharp line, and 

 look through it with the eye about R. 

 The line will appear doubled like m n,p q. 

 In like manner a black dot, or a luminous 

 point or aperture will appear double, as 

 e, o. If we cause a ray or pencil of 

 light R r to fail upon the surface of the 

 rhomb, it will be separated into two 

 rays or pencils ro, re, each of which 

 will emerge from the rhomb at o and e 

 in the directions oo', ee' parallel to Rr. 

 The ray R r has therefore suffered 

 B 



