ROTATORY POLARIZATION. 239 



solid angles, formed at the junction of the pyramid and prism, 

 are sometimes replaced by small secondary planes, which are 

 oblique with reference to the original planes of the crystal ; 

 and the form of the crystal is then called plagiedral. In the 

 same crystal these planes lean all in the same direction ; and 

 it is found that, when that direction is to the right (the apex 

 of the pyramid being uppermost), the crystal is right-handed ; 

 and that, on the contrary, it is left-handed, when the planes 

 lean in the opposite way. These relations between the ro- 

 tation of the plane of polarization, and the crystalline form, 

 have been found by M. Pasteur to hold in other crystals. 



Sir David Brewster subsequently discovered that the ame- 

 thyst, or violet quartz, is made up of alternate layers of right- 

 handed and left-handed quartz. This remarkable structure 

 may be traced in the fracture of the mineral ; for the edges 

 of the layers crop out, and give to the fracture the undulating 

 appearance which is peculiar to this mineral. But the 

 structure in question is displayed in the most beautiful man- 

 ner, when we expose a plate of this substance to polarized 

 light, 



The colours exhibited in polarized light likewise reveal 

 the existence of crystals of quartz penetrating others in va- 

 rious directions, when no striae, or other external appearances, 

 indicate their presence. 



(244) The phenomena of rotatory polarization in rock- 

 crystal have been accounted for by the interference of two 

 circularly polarized pencils, which are propagated along the 

 axis with unequal velocities, one revolving from left to right, 

 and the other in the opposite direction. 



For a plane polarized ray is equivalent to two circularly- 

 polarized rays of half the intensity, in which the vibrations 

 are in opposite directions. When a plane-polarized ray, 

 therefore, is incident perpendicularly upon a plate of rock- 



