Page:Optics.djvu/200

 It is very simple, and is sufficient for all experiment on polarization. It consists of a tube TT′, to the ends of which are fixed two collars which turn with sufficient friction to keep them fast in any position. Each of them bears a circular division which marks degrees. From two opposite points of their circumference proceed two brass stems TV, T′V′, parallel to the axis of the tube, and between them is suspended a brass ring AA, which can turn about an axis XX perpendicular to the common direction of the stems. The motion of the ring is likewise measured by a circular graduation, and it may be confined in any position by screws. When a plate of glass is to be exposed to the light, it must be fixed on the surface of the ring; then it may be placed in any situation whatever with respect to the rays of light which pass through the tube; for the collar, turning circularly round the tube, brings the reflecting plane into all possible directions, preserving a constant inclination to the axis, and this inclination may be varied by means of the proper motion of the ring round its axis XX. The graduated circle which regulates this motion should mark zero when the plane of the ring is perpendicular to the axis of the tube, and the divisions on the two collars should have their zeros on the same straight line parallel to the axis. In constructing the apparatus one should take care that these conditions are fulfilled; but it is of no great consequence that they be so exactly, as any error may be compensated by repeating each observation on both sides of the axis, and taking the mean of the numbers of degrees found in the two opposite positions.

If it be desired, for instance, to repeat Malus's experiment described above, a plate of glass must be placed on each ring, and they must be disposed so as to be inclined to the axis at angles of 35° 25′. Then the graduated circle of one of the collars must be brought to mark zero, and the other 90°, that the places of reflexion may be perpendicular to each other. The tube must then be secured, and a candle placed at some distance in such a position that its rays may be reflected by the glass along the axis TT′. This will happen when on looking through the tube the reflexion of the candle is seen in the first glass. Every thing being thus arranged, the reflected rays will meet the second glass at the same angle of 35° 25′; then according to the different positions given