. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 230 UNDULATORY THEORY OF fast as tho analyzer. Wlien the analyzer is in azimuth 90° from its original position, the dark radius is in azimuth 180° from its original position; when the analyzer has completed half a revolution, the dark radius has made a whole one. This singular fact is, however, easily explained. The analyzer suppresses that ray on whose plane of polarization it is crossed; and the planes of polariza- tion of the ray
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 230 UNDULATORY THEORY OF fast as tho analyzer. Wlien the analyzer is in azimuth 90° from its original position, the dark radius is in azimuth 180° from its original position; when the analyzer has completed half a revolution, the dark radius has made a whole one. This singular fact is, however, easily explained. The analyzer suppresses that ray on whose plane of polarization it is crossed; and the planes of polariza- tion of the rays in this small circle are, at opposite ends of every diameter, at right angles to each other. If QAPA' he the small circle of tangency, Q heing the point of contact with the circular section, or the foot of the normal, we have seen that the molecular movement from any other point of contact in this plane must be toward Q ; that is, the movement at q must be in the direction ^Q, ]. that at r in the direction ?Q, that at^ in the direction ,pQ, &c. Suppose the direction of molecular move- ment in the analyzer to be AA, parallel to cQa. The vibrations in the ray at Q have the same direction. The analyzer allows that ray therefore freely to pass; but it is crossed on the ray at P F'S- 70 whose direction of vibration is PQ. The radius, CP, will therefore be dark. Now let the analyzer take a position in azimuth A'A', parallel to a'a', tangent at r. Draw the diiimeter rCp. Draw (^sq perpendic- ular to rG. It will be parallel to a'a', and it will be the direction of vibration of a ray at q. The analyzer in the position A'A', is therefore in harmony with the ray q, and it is crossed on a ray whose molecular movements are in Qr, at right angles to Q^' Join Qr, Q/», rq. The angle (^pr is equal to the angle Q^r, since both stand on the same arc, rQ. And the triangle rqs is similar to the triangle Q^r, the one being right angled by construction, and the other because it is inscribed
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