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If, after having cut a rhomboid in the manner described, the eye be applied to the face, which is perpendicular to the axis, so as to receive only the rays which arrive in that direction, all the images of external objects will be single; they only undergo at their edges the diffusion which belongs to the general phænomenon of the decomposition of light by the unequal refractions.
But if the repulsive force, which produces the extraordinary refraction, really emanates from the axis, as the phænomena seem to indicate, it cannot disappear, except when the incident ray is parallel to the axis. The section, then, which we have described, is the only one in which a crystal prism can give a single image: this again is confirmed by experience, and we might avail ourselves of this character, to find the position of the axis in any piece of Iceland spar, not in the primitive form.
To return to our plate with parallel faces, cut perpendicularly to the axis. We have seen that the rays are not separated when they are incident perpendicularly; but when they enter obliquely, they ought to be separated, since they then form a certain angle with the axis, from which the repulsive force emanates. This is really what takes place; and moreover, for equal angles of incidence, the extraordinary refraction is the same on all sides of the axis, which shows that the repulsive force acts from the axis equally in all directions.
Many other crystallized substances, very different from the Iceland spar, exhibit like it a certain single line or axis, round which their double refraction is exerted symmetrically, being insensible for rays parallel to that axis, and increasing with their inclination to it, so as to be strongest for those which are at right angles to the axis. Crystals thus constituted are called crystals with one axis. For instance, quartz, commonly called rock crystal, has an axis parallel to the edges of the hexahedral prism, under the form of which it is generally found. But there is between its double refraction and that of the spar, this capital difference, observed by M. Biot, that in the spar the deviation of extraordinary rays from the axis, is greater than that of the ordinary, whereas in quartz crystals it is less. All crystals with one axis, that he has examined, have been found to possess one or other of these
