POLARISATION OF LIGHT. 



ourselves of the extravagant supposition jig. 74, ebcg, afh d, 

 of a crystalline embryo,~which, like that long diagonals EF, G 

 of animal and vegetable life, gradually 

 expands to maturity. The germ of 

 plants and animals is nourished by a 

 series of organs, of which, however re- 

 condite be the operation, we yet see the 

 action, and witness the effects ; but in 

 the architecture of apophyllite no subsi- 

 diary organs are seen. The crystal ap- 

 pears only in its state of perfection ; and 

 we are left to admire the skill which pre- 

 sided at its formation, and to profit by 

 the instruction which is so impressively 



faces, and parallel to the edge 

 F H . When we look perpendicularly 

 through the faces AEBF, DGCH, 

 the light will not pass through the above 

 planes, and consequently we shall see 

 only the two images of the object formed 

 by double refraction. But if we look 

 through any of the other faces, we shall 

 observe the two common images veiy 

 close together, then two secondary 

 images at a much greater distance, one 

 on each side of the two common images ; 



conveyed by such mysterious organisa- sometimes there are four, and sometimes 

 tions.' r * six secondary images, the secondary 



images being in two lines, one on each 

 CHAPTER XVII. side of the common images, and perpen- 



,, , , . 7 . , . ., T . 7 dicular to the line joining the common 



Multiplication of Images in Calcareous 



Spar- -caused by oppositely crystal- 

 lised Veins Explanation of the 

 Phenomena Method of imitating 

 them by artificial Combinations. 



images. Sometimes the secondary 

 images are double, triple, and quadruple, 

 so that when the interrupting planes are 

 numerous, the images are multiplied to 



THOSE who have been in the habit of 

 making experiments with calcareous spar 

 cannot fail to have observed that various 

 specimens, while they produce only two 

 images through two faces of the rhomb, 

 produce great numbers of coloured 

 images, by transmitting the light through 

 .the other faces of the rhomb. Upon 

 examining such specimens, it will be seen 

 that there are one or more planes pass- 

 ing through the specimen, and that light 

 is reflected from these planes, so as to 

 produce more than two images by re- 

 flection. This multiplication of images 

 was observed by Bartholinus and 

 Huygens, and was afterwards described 

 by 'Benjamin Martin, Dr. Robison, and 

 Mr. Brougham. 



The crystals of Iceland spar, which 

 possess this curious property, are inter- 

 sected with one or more planes, AB C D, 



Fig. 74. 



* Edinburgh Transactions, vol. is. p. 323. 



such a degree that heaps of them are 

 visible, sometimes varying in the inten- 

 sity and colour of the light, sometimes 

 vanishing, and sometimes re-appearing 

 by the inclination of the plate. These 

 phenomena are still further varied, if the 

 luminous object consist of polarised light. 

 The images are in general highly ^co- 

 loured, exhibiting the complementary 

 tints of polarised light, but in some spe- 

 cimens there are no colours at all except- 

 ing a prismatic tinge at the edges arising 

 from refraction. We have now a large 

 specimen before us, in which the rhomb 

 is so intersected with planes, that it 

 throws up a floating light like the finest 

 specimens of moon stone, but produces 

 none of the complementary colours of. 

 other specimens. 



Malus has explained the general phe- 

 nomena above described, by supposing 

 that the planes AB C D, &c.,are fissures 

 within the crystal, and that the colours 

 are those of a plate of air similar to the 

 Newtonian colours of thin plates. 



In this state of the subject Dr. Brew- 

 ster was led to the examination of the 

 phenomena. As the planes A B C D, &c., 

 are almost always extended to all the four 

 faces of the rhomb, and give exactly the 

 same colour at every part of their sur- 

 face, it was obvious that if a fissure oc- 

 casioned the phenomena, it must be 

 equally wide at every part of its surface, 

 an effect so extraordinary, that it could 

 not possibly take place. As the supposed 

 fissure extended to every surface of 

 the rhomb, it necessarily followed that 



