46 NOTICES OF THE MEETINGS [ March 28, 
M. Biot has been for forty years enriching chemico-physical science 
by a series of memoirs detailing the results of his study of these 
phenomena. He has there shown the value of this means of tracing 
changes in chemico-molecular constitution, 
M. Pasteur has carried forward this inquiry into a new channel 
by tracing a connexion between this property in substances, of 
circularly polarizing light, and their crystalline character. 
But as it would be impossible to explain the nature of his investi- 
gations, or their results, without a preliminary knowledge of the 
meaning of the terms “circular polarization,” and ‘‘ hemihedrism,” it 
was necessary first to enter a little upon the explanation of them. 
Accordingly a ray was explained as being a direction of light, 
having no relations to space which differed from each other in 
directions perpendicular to its length. Thus without complicating 
the subject, by using the language of the beautiful wave-theory, 
a ray might be imagined as a cylinder of minutest diameter 
but indefinite length. When such a ray is reflected at a cer- 
tain angle from glass or such like substance it is split into two; 
one going into, and through the glass if it be not opaque, the other 
being reflected from it. These two rays no longer possess the same 
“absence of sides” as the original ray. For the one has been as it 
were flattened down to a “ strip,” while the other has also been flat- 
tened similarly into a ‘‘ strip,” but the latter strip is at right angles 
in regard to its ‘flattened plane” to what the other is. A similar 
bifurcation of the ray is produced in the interior of what are called 
doubly refracting crystals. This bifurcation and flattening of the ray 
is termed “ plane polarization ”’ of it; and it is so far a true instance 
of polarity—as that the two rays have equal and similar properties in 
opposed directions. 
This was exhibited by the Lime-light. The double image of a 
small round hole formed by a crystal of Iceland spar was thrown on 
a screen, and each beam shown to be most capable of reflection in a 
plane in which the other was incapable of being reflected at all. The 
action of the tourmaline as a doubly refracting crystal which absorbs 
one of the rays was then explained ; and it was shown that the posi- 
tion of the tourmaline in which it intercepted one ray entirely, was 
exactly the position in which it gave the other ray free passage. 
The optic axis of a crystal was then defined to be a direction in it 
along which the light could pass through the crystal without under- 
going any change whatever. The central ray of a polarized beam of 
light, traversing a piece of cale spar along its optic axis, was shown 
to be intercepied or transmitted by a tourmaline, precisely as if the 
section of the crystal of cale spar were away. 
An exception was however stated to exist to this law of the 
neutrality of the optic axis. When a section of quartz. cut so that 
the beam could career along its optic axis, was put in the path of 
the polarized ray, it was found that instead of permitting the ray to 
be eclipsed by the tourmaline when this was placed in the position 
