Cuar. V., § 4.] 
the positions in which the light passed through un- 
changed. 
The theory of this simple yet admirable experi- 
ment is one of the happiest examples of Fresnel’s 
mechanical explanation of double refraction, But 
it was not attained by a single step, nor effected 
by a single hand. Malus had observed the fact of 
depolarization, and the existence of perpendicular 
neutral axes in the crystalline plate. Arago added 
to this the knowledge of the phenomena of colour, 
which Sir David Brewster also observed indepen- 
dently somewhat later. He also invented a par- 
ticular theory to explain them by what he termed 
moveable polarization. But it was not a success- 
ful effort. M. Biot assiduously studied the empi- 
rical laws of these periodic colours, and traced 
their dependence on the thickness of the interposed 
plate, according to a law similar to that of Newton’s 
rings. Young made an important step farther. He 
attributed the colour to the interference of the ordi- 
nary and extraordinary rays into which the incident 
light was divided on entering the crystalline plate ; 
and he showed by accurate calculation that the retar- 
dation of the slower-moving of the two rays during 
their passage through the plate, did in fact produce 
the difference of phase necessary for developing the 
tints observed by their reunion on leaving the plate, 
according to the usual laws of interference; and he 
OPTICS.—ARAGO. 
909 
selenite. But if the axis of the crystal be inclined, 
suppose 45°, to the plane of vibration of the incident 
polarized ray, the vibration is mechanically resolved 
into two, which are oppositely polarized, After tra- 
versing the thickness of the crystallized plate with 
the different velocities due to the motion of the ordi- 
nary and extraordinary rays, they are reunited at 
emergence, but in altered relative phases, so that by 
their union they form a beam no longer polarized 
(unless by an exception) in the same plane as at first, 
nor indeed plane-polarized at all, but most likely per- 
forming elliptical or circular vibrations, which, again, 
falling on the analyzer, are reflected or transmitted, 
or partly both, in a manner quite different from what 
would have happened to the light unchanged by crys- 
talline transmission. 
White light becomes coloured because the state of 
polarization of the emergent ray depends on the differ- the colour. 
ence of length of path for the two rays which under- 
went crystalline separation within the plate, and also 
on the length of a wave of light (for this determines the 
phase of polarization at emergence), But the wave 
of light varies in length for each colour, consequently 
every colour has its maximum under different cireum- 
stances, and if the incident light be white, the light 
falling after reflection on a screen will present mixed 
tints similar to those of Newton’s rings. 
This extraordinary property of a crystallized plate 
showed that this theory coincided with M. Biot’s 
rules But even this was not enough to explain the 
facts. It was not clear why the colours due to doubly- 
(which, in common light, appears equally transparent Similar 
and homogeneous in every direction), of modifying emt te 
light, or dyeing it with the most gorgeous colours, radiant 
Caseof refracting plates should not be seen without reaching when the plate is merely turned in its own plane, is heat. 
sahaly 5S, the eye through an analyzer, or cale-spar prism. To one of the nicest tests of the polarization of the light, 
larizea— Arago and Fresnel jointly we owe the important and has been used to detect the analogous polariza- 
are eee reply to this difficulty, which in fact forced upon the tion of radiant heat, and the concomitant phenome- 
resnel. 
latter the idea of transversal vibrations. Their joint 
experiments (mentioned in the last section, art. 488), 
had shown that oppositely polarized rays cannot in- 
terfere unless they have, first, acommon origin and a 
common plane of polarization; and, secondly, unless 
they be reduced to a common plane of polarization 
(that is, analyzed) before falling on the eye or the 
screen: so that the theory of the colours of crystal- 
lized plates is, briefly, as follows :— 
Polarized light is represented by transverse vibra- 
tions of ether, the particles moving all in one plane. 
The crystallized plate has, we will suppose (in order 
to take the simplest case) one axis of double refrac- 
tion, and the direction of that axis is in the plane of 
the lamina. When the vibrations of the light falling 
on the crystallized plate are either wholly parallel or 
wholly perpendicular to this axis, the light is trans- 
mitted without alteration, either as an extraordinary 
or as an ordinary ray, and it is then reflected or not 
by the analyzer, as would have been the case had it 
not been transmitted at all through the crystal of 
non of double refraction, which, except in the case 
of the heat accompanying the solar rays, has not 
yet been independently recognized.” 
Arago applied his discovery to the construc- 
tion of a polariscope, for estimating the feeblest 
amount of polarization ; and he used this instru- 
ment for some very interesting experiments on the 
polarization of the light of the sky (which is sun- 
light polarized by reflection from the atmosphere), 
and on that of different incandescent and reflecting 
surfaces, He also found that the moon and the 
tails of comets send light to the eye which is slightly 
polarized, thus betraying its borrowed origin. But 
that of the sun, being absolutely neutral, is only 
comparable (according to Arago) to the light arising 
from incandescent vapours, thus distinguishing the 
sun from a solid or liquid globe. 
We cannot do more than allude to Arago’s other 
optical papers and experiments, He was, probably, et 
the only Frenchman of his time who was well ac- Newton’s 
The explana- rings and 
quainted with Young’s discoveries, 
1 Quarterly Review, vol. xi. ; Miscell. Works, vol. i., p. 266. 
_* See Researches on Heat by the present writer. Hdinburgh Transactions, vols. xiii. and xiv. 
