larger now becomes the smaller one, and vice versâ. With increasing excentricity the elliptic vibrations, which are perpendicular to the initial ones, pass directly over them. During all this process, the direction of the vibrations did not change; supposing it to have been from left to right, it remained so. When however the second rectilineal vibration opens into an elliptic one, and the direction of the motion has become inverted, the vibration now takes place from left to right, supposing it to have been before from riglit to left. The vibrations then return through circular again into the initial vibrations.
The light proceeding from the cube was now circularly analysed, by means of the interposition of a lamina of mica of a proper thickness between the plate of Iceland spar and the analysing prism. The axis of this lamina lay so that the segments of the arcs were removed from the central point to the first and third quadrants. When the cube was yet unheated, its action was thus in direct opposition to its action in the first degree of its heating. When, proceeding from this point, the rings without the cross and with the black spot in the centre were formed, this spot, on the heat being increased, divided itself into two, which removed themselves from the centre into the second and fourth quadrants, and after having passed through the figure in the circular light, closed into a circle with the arcs proceeding from the first and third quadrants, so as to produce the system of rings with a bright centre, which would have been obtained at the very beginning by turning the polarizing prism 90°. The arcs, approaching nearer to the central point from the first and third quadrants, formed then the opposite circular figure, and united themselves at last in the centre into a black spot, whilst all the arcs closed themselves into circles. In this process, the phænomena before described of the linear analyses will again be easily recognised as a conditional element, without the necessity of particularly describing the alteration in form of the rings before they disunite into separate arcs.
To make circular light incident, is simply to add to the difference of phases produced by the heated cube a constant quantity, viz. or undulations ; that is to say, to alter the starting-point of the experiment. Having therefore inserted the lamina of mica between the polarizing prism and the heated cube, I obtained by linear analysis the phænomena first described, and by circular analysis those last described, beginning at another starting-point.
4. Phænomena in the different Colours of the Spectrum.
The foregoing experiments were made in incident homogeneous light, the length of whose waves was . In another part of the spectrum, however, has another value. Let represent this; and if
Vol. I.—Part I.
