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is observed in every other case, as shown in the Figure, the extraordinary ray lying always on the same side of the ordinary.
The inference to be drawn from this is, that there exists in the crystal some peculiar force which abstracts from the incident pencil a part of its molecules, and repels them towards B′. But what is this force? We shall soon see that it emanates, or seems to emanate from the axis of the crystal; that is, that if through each point of incidence there be drawn a line IA′ parallel to that axis, and representing its position in the first strata in which the pencil is divided, all the phænomena take place just as if there emanated from that line a repulsive force, which acted only on a certain number of luminous particles, and tended to drive them from its direction. This force always throws the rays towards B′, because they are always found on that side of the axis, under whatever angle of incidence they may have entered.
Let us follow up this idea, which does not appear repugnant to the few observations that have been made, and to verify it by a direct experiment, let us divide the crystal by two planes perpendicular to its axis, (Fig. 216.) so as to form two new faces abc, a′b′c′, parallel to each other. Now if we direct a ray SI perpendicularly to those faces, it will penetrate them in a direction parallel to the primitive axis of the crystal. Supposing then that the repulsive force emanates from that axis, it will be nothing in this case, and the incident rays will not be separated. This is, in fact, what takes place: there is in this case but one image.
It is even found, in making the experiment, that the image remains single when the second face of the plate is inclined to the axis, provided the first be perpendicular to it, and to the incident rays. This would happen, for instance, if only the first solid angle A of the primitive rhomboid were taken oft. The incident ray SI would continue its progress parallel to the axis, as before, and on emerging from the second surface, it would be refracted in one single direction, according to the law of ordinary refraction. Hence, we may conclude, conversely, that an incident ray R′I′, which passed out of air into such a prism under the proper angle of incidence, would be refracted in one single ray parallel to the axis, and emerge at I in the same manner. This again is confirmed by experience.
