332 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
Herschel, in all the needles of crystallized quartz whose lateral 
faces present triangular facets, directed obliquely to the axis, 
—a peculiarity which characterizes the variety called plagihe- 
dral,—the direction of the rotation, in the vicinity of these facets, 
is in constant relation to their direction; so that, like them, it 
must be an accidental result of the mode of grouping of the 
constituent molecules, and not a quality individually peculiar 
to them*, 
Although the justness of this consequence does not require 
that we should know how similar effects may be produced by 
* T possess two needles of rock crystal which present the very rare pecu- 
liarity of having plagihedral facets in opposite directions. Nevertheless, the 
plates perpendicular to the axis, which I have cut from them in the different 
portions of their length, have presented, in each crystal, a rotatory power of 
the same direction and of a constant energy. This energy appeared to me, 
moreover, not to differ from that which is found in other plates of the same 
mineral, when they are examined at some distance from their edges, where 
the uniformity of opposition of their laminz is generally disturbed by the in- 
fluence of external causes, which have arrested their transversal development 
and have placed a limit to it. But this observation does not destroy the reality of 
the relation observed by Sir John Herschel between the direction of the plagi- 
hedral faces and the direction of the rotatory action, when it is restricted to the 
specifications which he himself has assigned; for, in studying it in plates of 
amethyst, where the direction of the action is almost infinitely varied, he ob- 
served that the connection in question exists only, as a constant and general 
fact, for the transversal portions of the plates which are nearest to each plagi- 
hedral face; without our knowing why it takes place, why it ceases, or how 
far it may extend. If the reader wishes to know the elements of this discussion 
more in detail, he may consult Sir John Herschel’s memoir, inserted in the 
Philosophical Transactions of Cambridge for 1820, or the Comptes Rendus de 
l’ Académie des Sciences, for the first semestre of 1839, vol. vili. p. 683. We 
need only remark that the English philosopher had considered it correct to in- 
vert the significations of the assertions and signs which I had previously given 
to the two directions of the deviations; so that, instead of writing them as 
= 
being produced toward the right of the observer x , or toward his left a 
as they are seen, he writes them as they are not seen, placing the observer 
astride on the luminous ray, in a position opposed to his real position. This 
mode of statement, which would create continual chances of error in the tran- 
scription of the experiments, has been maintained in the general treatise of 
the same author on light; so that he attributes to the small number of active 
liquids, which I had up to that time observed, directions of deviation the inverse 
of those which I had assigned them. Other English philosophers have adopted 
the denominations imagined by their countryman. But as none of them, not 
even Sir John Herschel, have added, as far as I am aware, any new observa- 
tion to those which I had made, the direct mode of statement which I had 
introduced, and which | have continued to follow, has, I hope, generally pre- 
vailed. When the author of a discovery in physics has given a simple and 
faithful statement of it, which expresses its sensible characters, and which 
enables us to describe.their applications without risk of error, it would be just, 
and scientifically convenient, not to change that statement without serious 
motives; above all, care should be taken not to change it for a worse, when 
nothing new is added to the facts which it comprises. 
