an artificial Substance resembling Shell, 207 



a plane passing through A B, T* q 



we shall see two images together, °' 



as at A and D, fig. 3., and otlier b^ A^fto ©c 

 two, namely, one at B and the ^ ^^ ^ 



other at C. These images, B, C, separate by the inclination 

 of the plate exactly like those in fig. 1., and all the four. A, B, C, 

 and D, have the same absolute and relative polarization as the 

 four analogous images seen through the new substance, with 

 this difference only, that none of them are nebulous. 



If we conceive the "ii'icr, 4, 



vein E F to consist, 

 as in fig. 4., of a great 

 number of small cry- 

 stals, fir, b, c, di &c. in 

 place of one, the very 

 same effects will be 

 produced. 



When we look through the new substance, the multiplica- 

 tion of images takes place in "whatever azimnth we incline the 

 plate, the elongated images being always perpendicular to the 

 azimuth of inclination. Hence it follows, that these images 

 are produced by numbers of minute crystals lying in or near the 

 azimuth in which the plate is inclined ; and that these crystals 

 have their axes all inclined to that of the plate which contains 

 them, at the same angle as the vein E F, figg. 2 and 4, is in- 

 clined to the axis of the rhombohedron of Iceland spar. But 

 the remarkable result of these observations is, that in place of 

 one set of crystals, or sometimes three sets, which occur in cal- 

 careous spar in three different azimuths, we have here an infi- 

 nite numbei' of them lying in every possible azimuth, and these 

 so small in their dimensions that they cannot be recognised by 

 the most powerful microscopes, except as dark specks disse- 

 minated through the general mass ; and yet they indicate by 

 their action on light, not only their existence, but the position 

 of their axes, and their doubly refracting and polarizing struc- 

 ture, as unequivocally as if we could handle them, and cle'we 

 them, and place them upon the goniometer. 



It may now be asked why the images are nebulous, and not 

 distinct as in calcareous spar. The reason is, that the sub- 

 stance is imperfectly crystallized like the agate, mother-of- 

 pearl and other bodies in which the doubly refracting force 

 separates the incident light into two oppositely polarized pen- 

 cils, which are not perfectly equal and similar, but which dif- 

 fer from each other, sometimes in the intensity of their light, 

 sometimes in the distinctness of the image, sometimes in the 

 nature or brightness of the colour, and sometimes in more 



