CIRCULAR POLARIZATION OF LIGHT. 81 



larger now becomes the smaller one, and vice versa. 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 micayof a proper thickness be- 

 tween 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 with- 

 out the cross and with the black spot in the centre were formed, tliis 

 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 them- 

 selves at last in the centre into a black spot, whilst all the arcs closed 

 themselves into circles. In this process, the phaenomena before described 

 of the linear analyses will again be easily recognised as a conditional ele- 

 ment, 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, — 7 — or 

 2/1 + 1 . - 4. 



— 7 — undulations ; that is to say, to alter the starting-point of the ex- 

 periment. Having therefore inserted the lamina of mica ^ between the 

 polarizing prism and the heated cube, I obtained by linear analysis the 

 phaenomena first described, and by circular analysis those last described, 

 beginning at another starting-point. 



4. Phenomena in the different Colours of the Spectrum. 



Tlie foregoing experiments were made in incident homogeneous light, 

 the length of whose waves was X. In another part of the spectrum, 

 however, X has another value. Let X^ represent this; and if 

 o — p = wj X, o — e =: m^ X^, 



^'()^. I. — Part I. c; 



