902 
Essayon Sciences for the theory of double refraction, Malus 
double re- wrote a second paper, which was crowned, but which, 
fraction. < ¢ A 
however admirable as a specimen of mathematical 
address, added little to what was previously known. 
(479.) Malus had already, in the end of 1808, announced 
Huygens’ g property of light which, if not absolutely new, was 
discovery 
of palariss- entirely so with reference to the circumstances in 
tion by calc Which it was produced. The polarization of light 
spar. was in reality discovered by Huygens previous to 
1680. He had observed that the two rays into which 
common light is divided in passing through Iceland 
spar have a singular diversity of character, which 
Newton afterwards described as an opposite polarity. 
Huygens showed that if two rhombs of doubly re- 
fracting spar are laid symmetrically one upon the 
other, the “ extraordinary” ray yielded by the first is 
extraordinarily refracted by the second, and the “ordi- 
nary” ray from the first is ordinarily refracted by the 
second. But when we revolve the upper rhomb 
as it lies upon the lower one through a right angle, 
a remarkable change appears. The extraordinary 
ray escaping from the first is now ordinarily re- 
fracted by the second, and vice versa, so that the 
qualities of the two rays differ but only so far as this, 
that either may be assimilated to the other by making 
it (or the crystal from which it derives its properties) 
revolve round ninety degrees. A definite notion of 
such a distinction may be formed by imagining a 
musical string vibrating at one time in a vertical, at 
another in a horizontal, plane. If we could possibly 
imagine light to consist of vibrations of this descrip- 
tion, the two rays of Iceland spar might be conceived 
the one to vibrate in a plane passing through the 
axis of the crystal, the other in a plane perpendicular 
to that. Such light might truly be said to have ac- 
quired the property of having sides. In the language 
of Newton, it is polarized, 
The discovery of Malus consisted in showing that 
light may acquire properties identical with those of 
either ray yielded by refraction through Iceland spar, 
by the very simple process of simple reflection at a par- 
ticular angle from any transparent body. Thus for a 
surface of water he found this angle to be 52° 45’ with 
the perpendicular, and for glass 54° 35’. The reflected 
light in either case has exactly the property of the or- 
dinary ray transmitted bya crystal whose principal sec- 
tion (that is, a section passing through the axis of the 
crystal) is parallel to the plane of reflection. Con- 
sequently this light will be acted on by a doubly re- 
fracting crystal placed in its way precisely as if it 
had emerged from a similar crystal; and, on the 
other hand, if the two rays emerging from a crystal 
be incident on water or glass as above mentioned, the 
one will be copiously reflected from the surface, whilst 
the other will not be reflected at all, but pass entirely 
into the transparent substance. Farther, as might 
be expected, light thus polarized by reflection, when 
(480.) 
Malus dis- 
covers po- 
larization 
by reflec- 
tion. 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
it falls on a second similar reflecting surface at the 
same angle as before, will be copiously reflected if the 
planes of reflection coincide, but will refuse to be 
reflected in an appreciable degree when the planes of 
reflection are perpendicular. 
This phenomenon was detected by Malus byeasually 481.) 
observing that a ray from the setting sun reflected Occasion 
from a distant window, and viewed through a piece® 
of Iceland spar, afforded but a single image in two 
positions of the latter. The experiment attracted uni- 
versal attention, and became the germ of a series of 
optical discoveries almost unprecedented for their 
beauty and variety ; yet most of the experiments may 
be made quite as well when light is polarized by the 
method of Huygens as by that of Malus. Neverthe- 
less the former had remained a sterile fact for 130 
years. Upon such trifling circumstances does the 
progress of knowledge often depend. 
Malus survived his discovery only four years, and 4g2.) 
saw but the borders of that land of promise which he Law of the 
had pointed out to others. A few results he however °*i"¢s- 
obtained, which are worthy of notice. Thus he found 
that in every instance where light is polarized in any 
plane there is also produced a certain proportional 
amount of light polarized in the perpendicular plane, 
Arago afterwards proved the very important fact, that 
these two portions are universally equal. Huygens 
had shown in the experiment of the two rhombs, that 
when their positions are neither symmetrical nor 
perpendicular, each ray emerging from the first is 
duplicated by the second. When the principal sec- 
tions of the rhombs are inclined 45°, the duplicated 
rays are equally bright ; as they approach parallelism 
or perpendicularity, one pair of the rays brighten and 
the other pair is enfeebled. Malus ascertained the 
law of change of brightness, which is the same for 
rhombs of spar or for plates of glass whose planes 
of reflection vary whilst the angle of reflection re- 
mains constant. In either case the intensity of the 
light varies as the square of the cosine of the angle 
formed by the principal sections of the crystals or 
the'planes of reflection of the plates. This impor- 
tant law, the best established in photometry, has been 
applied by Arago to the measurement of light in 
many instances, but the details were unfortunately 
not made public before his decease. 
To Malus is also due the discovery of the polariza- _(483.) 
tion of light by common refraction. When light is in- Seapets 
cident on glass or water, the refracted beam contains °’°""** 
precisely as much polarized light as the reflected beam, 
but oppositely polarized. The metals were at first 
believed by Malus to polarize no appreciable quan- 
tity of light. He afterwards found that at great in- 
cidences the reflected light is partly polarized." He 
likewise ascertained the fact of the “depolarization” (as 
it was termed) of light by many crystals, and also by 
organized substances, such as hair, horn, and whale- 
1 See an interesting letter from Malus to Dr Young in Thomson’s Annals, vol. iii. 
