POLARISATION OF LIGHT. 



25 



in nitre about 8 at the eye; but within 

 the substance of the crystal the axis forms 

 an angle of only 5 20'. 



In crystals such as Nitre, Carbonate of 

 Lead, Arragonite, where the inclination 

 of the axes of no polarisation, or of the 

 resultant axes, as they will afterwards be 

 found to be, is small, we can easily see 

 at once the two systems of rings sur- 

 rounding their two poles, and the two 

 poles themselves surrounded by the same 

 ring; but when the inclination of the 

 resultant axes is great,asinTopa;z,3/zra, 

 Feldspar, fyc. we can only see at once 

 the system of rings round each pole. In 

 order to do this advantageously, it is 

 convenient to grind and polish a face per- 

 pendicular to the axis passing through 

 that pole. In mica and topaz, however, 

 this is not necessary, owing to the facility 

 with which these minerals cleave in 

 planes equally inclined to the two re- 

 sultant axes. 



If AB, for example, (fig. 31,) is a 

 plate of topaz cut or split perpendicular 

 to the axis of the prism; then if we 



Fig. 31. 



Fig. 32. 



place this in the apparatus, fig. 21, so 

 that the ray of polarised light passes 

 along D C rfN, we shall see a system of 

 rings' like that in fig. 32. 



In like manner, by transmitting the 

 polarised li^ht along E C e M, so that 

 DCP=ECP, we shall see the very 



Right Prism 



Oblique Prism . . . 



Octohedron 



The following is a list of crystals, with 

 a double system, in which Dr. Brewster 

 measured the inclination of the resultant 

 axes within the substance of the crystal 

 The measures were carefully made, but 



same system. Hence DC, EC are the 

 resultant axes or "lines of no polarisation, 

 and the angle DCEis equal to 121 16'; 

 but the real inclination of the resultant 

 axes within the crystal, or e C d, is about 

 65. 



Hitherto we have supposedthat the ana- 

 lysing plate, C,fig. 21, is fixed, while the 

 plate of nitre or topaz revolves. But if 

 we suppose the nitre or topaz fixed in 

 any of the positions which give the phe- 

 nomena shown in fig. 27 30, and then 

 turn round the plate C, we shall see in 

 the azimuths of 90 and 270, a comple- 

 mentary system of rings, in which the 

 black cross is white, the dark pails light, 

 the red green, and so on, as described in 

 our account of the Uniaxal System of 

 Rings. 



The crystals which possess a double 

 system of rings, are very numerous ; and 

 we must refer the reader to the long list 

 of them given by Dr. Brewster in the 

 article Optics, in the Edinburgh Ency- 

 clopaedia. He found that all crystals 

 have a double system of rin^s, which 

 belong to the prismatic system of Malus, 

 or whose primitive forms are : 



Base a Rectangle 

 . Base a Rhomb 



Base an Oblique Parallelogram 

 . Base a Rectangle 



Base a Rhomb 

 . Base an Oblique Parallelogram 



Base a Rectangle 

 . Base a Rhomb 



some of them are only approximate re- 

 sults, and will admit of considerable cor- 

 rection by employing better specimens 

 than he was able to procure. Many of the 

 measures, indeed, were taken with veiy 



