430 Royal Institution. 



It was shown, however, that another specimen exhibited these colours 

 in the reverse order of red, orange, green, blue, plum-colour, red ; 

 in which order the former specimen of quartz produced these colours 

 when the tourmaline was turned in the opposite direction. Hence 

 these are termed right- and left-handed polarizations. The whole of 

 these phsenomena were attributed to a complicated set of movements 

 of the light within the crystal, the residlant of wliich was practically 

 a rotation of the plane in which the ray was capable of being re- 

 flected, so that the thicker the crystal, the further round the tour- 

 maline had to be turned to permit the ray to pass it, or to be eclipsed, 

 as the case might be. The opposite order of the colours was ex- 

 plained by the fiction of supposing the one to be the effect of a left- 

 handed thread to the screw and the other of a right-handed thread 

 _ characterizing the spiral in which the plane of polarization was sup- 

 posed to rotate. Of course this was only a popular way of explain- 

 ing the phsenomenon, it being really due to a more complicated series 

 of movements which were explained by Fresnel in the most trium- 

 phant manner by the wave-theory. 



The colour was accounted for by the idea of the red following a 

 longer spiral (having a coarser thread to the screw) than that of the 

 orange, this than the yellow, and so on up to the violet. Without 

 the tourmaline in front all would emerge and form white light ; but 

 the tourmaline only allows such rays to pass it as are capable of 

 passing it in its particular position ; i. e. only such, the rotation of 

 whose plane has brought them round to the position of the plane in 

 which the tourmaline lets the light through. 



The singular fact of amethyst being a combination of alternate 

 layers of right- and left-handed quartz was then exhibited, both by 

 throwing the image of the alternate layers on the screen, and after- 

 wards by showing that the general effect of a traversing polarizing 

 beam was to produce a neutrality of action. Other substances, 

 however, produce phjenomena of circular polarization. Uncrystal- 

 lized, fused tartaric acid, and barley-sugar, &c. produce them ; and 

 these bodies when dissolved, and many more in the form of liquids 

 also, do so, some of which were exhibited. But the silica of which 

 quartz consists entirely loses this property when divested of its cry- 

 stalline character, whether artificially or in its natural state as chal- 

 cedony, opal, &c. All other bodies retain it so long as their chemi* 

 cal molecule retains its individuality of character. 



The next point to be made clear, was the meaning of that form 

 of crystallographic development called " hemihedrism." Haiiy's 

 great law was, that similar edges or angles were always similarly 

 modified. The nature of similarity in edges or angles was then 

 pointed out, and the general idea of many crystallographers of a sort 

 of nucleus or primitive form existing on which the crystal was formed, 

 was explained, as also the nature of the development of such a cry- 

 stal by the modifications placed on the edges and angles according 

 to the law before mentioned. But the exception to that lavi^ was 

 not less remarkable for its generality of character than the law itself. 

 This exception consists in the fact that very often crystals are found 



