62 



POLARISATION OF LIGHT. 



the slightest blow would produce a sepa- 

 ration "of the two portions of the crystal 

 between which it lay. Dr. Brewster, 

 therefore, tried to produce such a cleav- 

 age ; but he found this impracticable, 

 and upon grinding down the crystal, and 

 removing the calcareous spar with a sharp 

 knife, till he reached the supposed fissure, 

 he found that there was no such breach 

 of continuity in the mineral, but that the 

 adhesion of the molecules was exceed- 

 ingly powerful at the very place where 

 the fissure was supposed to exist. 

 Upon more minute examination he found 

 that the phenomena were all owing to 

 veins or thin crystals of calcareous spar, 

 which interrupted the regular formation 

 of the minerals, or, what is the same 

 thing, that the rhombs which produced 

 the multiplication of images were hemi- 

 trope crystals of calcareous spar. This 

 opinion is capable of the most rigid de- 

 monstration. If we cleave the crystal, 

 fig. 74, in the direction A E B F or 

 D G C H, we shall find that the edges 

 A B e b, af of the thin crystal are not 

 coincident with the general surface, but 

 present each a face inclined 141 44'; 

 while a cleavage parallel to all the other 

 faces exhibits no such crystalline face, 

 the cleavage of the veins being coincident 

 with the cleavage of the general crystal. 

 These facts determine the exact position 

 of the axis of the vein, and by cutting 

 two faces on the crystal, perpendicular 

 to this axis, we shall observe the system 

 of rings belonging to the vein itself. 



The cause of the multiplication of the 

 images will be understood fromyzg-. 75, 

 where M N is the section of the vein or 

 crystal of calcareous spar placed within a 



Fig. 75. 



the vein M N is not in the position where 

 double refraction does not take place* ; 

 but as the vein is so thin as to produce 

 colours by polarised light, each of the 

 pencils c e and df will consist of two com- 

 plementary colours, depending on the 

 thickness of the vein, and the inclination 

 of the polarised rays, b c, b d to the axis 

 of the vein. These double pencils will 

 emerge at e,f from the vein, and be divi- 

 ded, as in the figure, into the rays em, en, 

 f o,fp, the colour of the pencils e n,fo, 

 being complementary to those of em, fp. 

 The rhomb of calcareous spar, shown 

 in Jig. 75, is equivalent to the polarising 

 apparatus shown in fig. 39, the light 

 being first polarised by the rhomb 

 A M N C, the vein M N being the thin 

 crystallised vein shown at D E F G in 



rhomb whose principal section is A B C D. 

 A ray, R b, incident at b, being refracted 

 doubly at b, will enter the plate of spar 

 at c d, where each pencil will surfer 

 double refraction a second time, because 



the rays b c, b d not much inclined to the 

 axis of M N, the colours are recognised 

 as portions of the system of coloured 

 rings which surround the axis of M N. 



In order to give ocular proof that the 

 multiplication and colour of the images 

 are produced by the causes above ex- 

 plained, Dr. Brewster divided rhombs of 

 calcareous spar, and inserted between 

 them, or into grooves cut in them, plates 

 of calcareous spar, or thin films of sul- 

 phate of lime and mica, and was able to 

 reproduce all the phenomena displayed 

 by the natural compound crystal. 'The 

 phenomena admit of many interesting 

 variations, by interposing several thin 

 films in different azimuths round the 

 polarised pencils b e, b d, and at different 

 inclinations to the axis of the principal 

 rhomb. Some of these phenomena have 

 been already referred to in p. 22, at the 

 end of Chapter VI. 



CHAPTER XVIII. 



Influence of an uniform Heat upon doubly 

 refracting Crystals upon Calcareous 

 Spar Sulphate of Lime Curious 

 experiment with Sulphate of Potash 

 and Copper, with the Hydrous Sul- 

 phates of Magnesia and Zinc Re- 

 markable effect of Heat on Sulphate of 

 Lime and on Glauberite. 



THE very curious subject of th& influence 

 of heat upon double refraction has been 

 recently investigated by Professor Mit- 



* See Chap. iv. fig. 19, p. H. 



