80 ~ DOVE’S EXPERIMENTS ON THE 
the ring m the screw was withdrawn a turn, in order that the motion of 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 re- 
ceded from the central point, whilst the first and third approached it. 
After some time the dark ares of the odd quadrants exactly corresponded 
ith 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 ares, 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 interven- 
ing 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 the 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 
ares 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 phenomena were observed in regular succes- 
sion during the cooling of the apparatus *; consequently the action of the 
glass, becoming gradually heated from below upwards, upon the incident 
light, is as follows. The particles of ether, which at first vibrate recti- 
linearly, 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 phaznomena 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 
phzenomena 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. 
oe 
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