1851.] OF THE ROYAL INSTITUTION. 47 
to eclipse it, that ray on the contrary fell on the screen endowed 
with beautiful colour ; and furthermore that the revolution of the tour. 
maline induced the most brilliant succession of colours, in the order, 
in the instance exhibited, of red, plum-colour, blue, green, orange, red. 
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 phenomena were attributed to a complicated set of movements 
of the light within the crystal, the resultant of which 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 phenomenon, it being really due to a more complicated series 
of movements which were explained by Fresnel in the most 
triumphant 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; 7.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 phenomena of circular polarization. Uncrystallized, 
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 crystalline 
character, whether artificially or in its natural state as calcedony, 
opal, &c, All other bodies retain it so long as their chemical 
molecule retains its individuality of character. 
The next point to be made clear, was the meaning of that form 
of crystallographic developement 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 
