the ring the screw was withdrawn a turn, in order that the motion of the rings, either away from the central point or towards it, might be the more easily observed.
The lamp having been lighted, the black cross began directly to open in the centre; the circular arcs in the second and fourth quadrant receded from the central point, whilst the first and third approached it. After some time the dark, arcs of the odd quadrants exactly corresponded with the bright vacant spaces of the even ones; the light was circularly polarized,and the difference of path was a quarter-undulation. Whilst this was going on, with the exception of the points proceeding from the centre which remained black, the dark cross had become brighter and brighter. When it had entirely disappeared, the arcs, growing shorter at their ends, had gradually advanced, so that the two black spots proceeding from the centre formed with the parts approaching each other from the two other quadrants, the inner ring, separated by four bright intervening vacant spaces. All the other rings were in the same state. The figure given by the Iceland spar had thus changed, precisely as if the polarizing prism had been revolved 90°; the light was therefore polarized linearly and perpendicularly to tlie plane of primitive polarization: the difference of path of both rays was a half-undulation. On a further heating, as the difference of path became three quarters of an undulation, the light was again circularly polarized, with the difference, however, that now the rings in the first and third quadrant were the nearest, those in the second and fourth the more distant; in which case the direction of the motion of the arcs in the single quadrants naturally remained the same. Finally, when the difference of path amounted to an entire undulation, the white cross became darkened into a perfect black; the arcs previously separated closed in whole circles; the light was polarized rectilinearly in the same direction as at the beginning of the experiment. The lamp was now removed and the opposite phænomena were observed in regular succession during the cooling of the apparatus[1]; consequently the action of the glass, becoming gradually heated from below upwards, upon the incident light, is as follows. The particles of æther, which at first vibrate rectilinearly, begin to open into ellipses, the excentricity of which diminishes continually, until they become circles. The axis which at first was the
- ↑ Precisely the same succession of phænomena may naturally be produced by the gradual increase of pressure or its relaxation. With the plates, however, which I had employed I was able to carry it only as far as a difference of path of three quarters of an undulation in the proximity of the points of action of the screw. On applying a stronger pressure the plates broke. Now it is evident that when a cooled glass plate, which in white light exhibits a regular series of colours proceeding from black, is interposed, in homogeneous light the same phænomena will be observed in the plate of Iceland spar, if it be slowly moved along before the aperture of the polarizing prism. The thicker the plate the nearer to each other are the differently-acting vacant spaces.
