910 
onthe ro- tion by the doctrine of interference of the colours 
tatory ac- of Newton’s rings received an important confir- 
MATHEMATICAL AND PHYSICAL SCIENCE. [Diss. VI. 
ferred to, arts. 488 and 506; but to Arago alone isin dense 
due the ingenious idea of interposing a thin slip of media 
tion of proved by 
quartz, 
(512.) 
mation from an experiment of Arago’s, which proved 
them to arise from the mixture of the pencils 
of light reflected at the two neighbouring sur- 
faces. He pressed a lens of glass against a plate of 
metal, in which case the central spot is white or 
black when light polarized perpendicularly to the 
plane of incidence is reflected at an angle greater or 
514. 
less than the polarizing angle for glass; and the rings obtaining of a still more direct proof of this fact, which met 
vanish altogether at the polarizing angle ;—results he considered as a crucial one between the rival hypo- ment to the 
which have been found conformable to the undulatory theses of Newton and Huygens. In his last years he by it ous 
theory.2 He also discovered the peculiarity of the 
rays transmitted along the axis of a crystal of quartz. 
These depolarize light, or produce colours similar to 
those of crystallized plates, varying according to a 
well-marked law with the thickness of the plate. 
The most singular fact is, that by turning round the 
analyzing plate, no position of neutrality is found, 
but a series of colours similar to those of Newton’s 
scale succeed one another, Arago showed that this 
effect is due to a rotatory motion of the plane of polar- 
ization within the crystal. The rotation is greater 
for the violet than the red ray; this was shown by 
M. Biot, who also discovered that in some specimens 
the rotation takes place from right to left, in others 
from left to right—a peculiarity connected with cer- 
tain crystallographic modifications, as was first 
shown by Sir John Herschel. 
MM. Seebeck and Biot discovered an analogous 
Rotation of property in oil of turpentine, and in various saccharine 
the plane 
of polariza- 
tion in 
fluids. 
(513,) 
fluids, an observation which, in many cases, allows the 
substitution of an instantaneous optical, for an operose 
chemical test. Fresnel has shown that the phenomena 
of quartz may be represented on the supposition that 
the rays traversing the axis consist of two rays circu- 
larly polarized in opposite directions, and travelling 
with different velocities; and Mr Airy succeeded in 
calculating, bythe aid of this fundamental hypothesis? 
a number of most beautiful and complicated pheno- 
mena, such, for example, as those which occur 
when plates of right and left handed quartz are 
superposed. Maccullagh has shown how the ge- 
neration of elliptical or circular vibrations may be 
deduced from the general equations of motion, but he 
has not invented a mechanical theory to explain 
them. This is a point of the very highest interest, 
inasmuch as Dr Faraday has succeeded, for the first 
time, in inducing artificially in a substance the power 
of rotating the plane of polarization by the presen- 
tation of it to the poles of a most powerful magnet.® 
Arago’s experiments on the non-interference of 
Retarda- rays of light oppositely polarized, being undertaken 
tionoflight in conjunction with Fresnel, have been already re- 
mica or blown glass in the path of one of the inter- 
fering pencils, and observing the displacement of the 
interference-bands, which is always towards the side 
of the interposed slip, showing that the movement 
of the wave has been slower within the denser me- 
dium. 
Arago continued to attach great importance to the 
Arago. 
had the satisfaction of witnessing the accomplishment cault. 
of it, with the result he anticipated, and by a method 
which he had himself indicated. In 1838, he had al- 
ready indicated the application of Mr Wheatstone’s 
beautiful invention of therevolving mirror,*asameans 
of measuring intervals of timeincredibly short, in order 
to compare the velocity of light in air, and ina corre- 
sponding length ofwater. He even caused an apparatus 
to be partly prepared, but we have seen that Arago’s 
forte was rather in suggesting than in completing re- 
searches. After his increasing failure of sight ren- 
dered it physically impossible that he should ever 
realize his own idea, it was skilfully adopted by M. 
Foucault (the author of the admirable experiment 
with the pendulum, demonstrating the earth’s mo- 
tion®), who, by an ingenious combination of fixed and 
revolving mirrors, succeeded in 1850 in demonstrating 
the retardation of light in a tube of water only 64 
feet long, and with a velocity of rotation of the move- 
able mirror not exceeding 200 turns in a second (a ra- 
pidity four times less than had already been obtained 
by Mr Wheatstone). The rotation was produced 
by means of the Siréne of M. Cagniard de la Tour, 
acting by steam. The velocity was thus raised to 
1000 revolutions. It was afterwards, however, carried 
by MM. Fizeau and Breguet to 2000 revolutions. It 
will be understood, from the account of the method, 
as applied to the measurement of the velocity of elec- 
tricity, in another chapter, that the retardation is 
shown by the displacement of the image of a minute 
object seen through the water, relatively to the image 
of the same object seen in air, If light moves faster 
in water (as Newton imagined), the displacement of 
the water-image will be (let us say) to the right; but 
if slower (as Huygens and Young believed), it will 
be to the left, The calculated displacement, with 
800 revolutions in a second, was *004 inch on the 
first supposition, and +003 in the opposite direction 
in the second, quantities easily visible with a high 
magnifying power. The result, as has been stated, 
confirmed Arago’s original experiment of 1815 on 
the displacement of the interference fringes. 
1 Sir W. Herschel first formed Newton’s rings between glass and metal. Arago’s experiment was reproduced (unknowingly) 
by Mr Airy in 1831, who first explained it fully in the undalatory sense. Camb. Trrans., vol. iii. 2 . 
? With this addition, that rays inclined to the axis are elliptically polarized, and that with a greater ellipticity as the inclina- 
tion increases. 
3 See the chapter on Electricity, § 5. 
4 See Electricity, § 6. 
5 See the chapter on Astronomy, Art. (258). 
