914 
curious phenomena to anything like a law. It was 
MATHEMATICAL AND PHYSICAL SCIENCE. [Diss. VI. 
Malus, who at first believed Laws of 
reflect is very peculiar. 
metallic 
Law of the tion, 
tints. 
evident almost from the first, that the axes in ques- 
tion (which he termed axes of no polarization) are 
only the resuitants of remoter fundamental actions of 
the crystalline constitution. For example, these 
axes vary in position according to the colour of the 
light used to display them; their position within the 
crystal varies (as was shown by Mitscherlich) with 
the temperature of the body, nor is it obviously 
related to any of the geometrical lines of erystalliza- 
Sir David Brewster succeeded in finding the 
law of the tints expressed upon the surface of a 
sphere of which the directions of the two axes form 
diameters. M. Biot expressed the law more ele- 
gantly by saying, that the tint developed by a biaxal 
crystal in any ray, is proportional to the product of 
the sines of the angles which the ray in question 
makes with the two optic axes. The tints are con- 
sequently arranged round the two poles of the axes, 
in a series of curves resembling the figure 8, having 
each this property, that the product of the sines of 
the angular distances of each point of one curve from 
that they were incapable of polarizing it in any de- 
gree, afterwards changed his opinion, and inge- 
niously suggested that whilst transparent bodies 
reflect, at the polarizing angle, light polarized only in 
one plane, metals reflect rays oppositely polarized 
and then mixed. Sir David Brewster has the merit 
of having, after several unsuccessful attempts, de- 
duced the leading empirical laws of metallic polari- 
zation, having been partly guided (as he states in his 
paper in the Philosophical Transactions for 1830) 
by Fresnel’s remarkable experiments on circular 
polarization produced by total reflection in glass 
(see Art. 491). Having found qualities somewhat 
analogous in light reflected one or several times from 
metallic plates at various angles depending on the 
nature of the substance, he gave to the light so re- 
flected the name of elliptically polarized light. It 
was afterwards satisfactorily proved by Mr Airy that 
the light so named by Sir D. Brewster is in fact 
identical in its qualities with the elliptically polar- 
ized light of Fresnel. 
reflection, 
The subject of metallic polarization is rather too (534.) 
abstruse to be explained in a popular way; and the Metallic 
phenomena produced with depolarizing plates of dif}? “"'* 
the two poles is equal to a constant quantity. Such 
curves are called lemniscates, and are beautifully 
seen in nitre, especially when viewed by homoge- Son, 
(532.) 
neous light. 
A series of researches of the most elaborate 
Relation of description led Sir David Brewster to this addi- 
optical cha- tional and admirable discovery, viz., that the optical 
racters to 
erystalline 
forms. 
(533.) 
characters of single refraction, double refraction 
with one axis, and double refraction with two axes, 
have reference invariably to the primitive crystalline 
form of the mineral, and that the complexity of the 
optical character is as invariably related in degree to 
the complexity of the crystalline figure. Cubical and 
regularly octahedral crystals (as rock salt and fluor 
spar) being possessed of perfect symmetryin three prin- 
cipal directions, possess also simple refraction. Crys- 
tals with one predominant line or axis of symmetry 
—as rhombohedrons, octahedrons with square bases, 
right prisms with square or hexagonal bases—have a 
single axis of double refraction. Such, for instance, are 
Iceland spar, zireon, ice, beryl. Finally, all crystals 
unsymmetrical in the three principal directions, in- 
eluding prisms and octahedrons whose bases are 
not square, and those which are oblique, have two 
axes of double refraction, The rarity and minute- 
ness of many crystals, the difficulty of cutting them, 
and when cut, of detecting their optic axes, evi- 
dently made the research one of extreme labour, yet 
highly remunerative, not only through the discovery 
of the general principle, but by the vast amount of 
beautiful and varied optic displays witnessed in the 
course of it, Sir David Brewster was nearly, if not 
quite, alone in this research, and after a short resis- 
tance on the part of some mineralogists, his principle 
of discrimination of primitive forms of crystallization 
by optical characters has been perfectly established. 
IV. The action of metals on the light which they 
ferent metals are not so well known as, according to 
their discoverer, they deserve tobe. My limits only 
permit me at present to state that the care and accu- 
racy of Sir D. Brewster’s results are unquestionable; 
that they have formed almost the sole data upon 
which M, Cauchy and other mathematicians have 
based their theories of metallic reflection ; and that, 
by generalizing the more limited views entertained by 
Fresnel as to the constitution of media and the nature 
of reflected light, they have been mainly instrumental 
in fixing the later views of optical writers as to the 
precise phenomena of polarization as produced, not 
only by metals, but by other substances, To these 
views I shall briefly advert in the next section. 
The laws of the reflection of light at crystallized (535.) 
surfaces have also been studied by Sir D. Brewster. 
In this case observation is still in advance of theory. 
V. Of Sir David Brewster’s experiments on the 
absorption of light we must speak much more Absorption 
briefly. White light is coloured or analyzed by® light. 
refraction (as in a prism); by simple interference, as 
in Newton’s rings; by double refraction combined 
with polarization. But it is also decomposed in a 
way which, primarily at least, seems different from 
all these,—by passing through coloured, or ratner, 
colouring media, whether solids, liquids, or gases, 
as red glass, ink, chlorine. This, the most familiar 
mode of coloration, is the most difficult to account 
for, and has been (on account of its obscurity) less 
studied than the others. In some instances, the 
complementary colour (that which, added to the 
transmitted tint, makes up white light) is entirely 
absorbed or lost; in other cases it is reflected at or 
near the first surface of the medium, Sometimes 
