560 OPT 
tals: tbefe different modifications of the light apparently 
proceeding from fome unknown caufe, which prevents 
the ray penetrating the fecond rhomboid when placed in 
certain pofitions with refpeCt to the firft. 
. This is a iimpleand eafily-exhibited cafe of polarifation. 
Again : if the luminous objeCt (for inftance, the flame of 
a candle) be refleCfed from water at an angle of 52'-! 45', 
and viewed through a cryftal of Iceland fpar, in one pofi- 
tion of the cryftal two images are feen : let the cryftal be 
turned through an arch of 45 0 , and one of the images 
w-ill be found to difappear ; after revolving through 90 0 , 
it becomes vifible again ; and foon alternately through the 
other parts of the whole circuit. The light has therefore 
evidently acquired the lame properties as if it had been 
tranfmitted through a rhomboid of fpar, previous to the 
fecond cryftal being applied : that is, in being reflected 
from water at the angle we have ftated, it has become 
polarifed. It has been found, that the angle at which light 
mull be incident upon the reflecting furface of .a tranfpa- 
rent body, in order to be polarifed in the moft diftindt 
manner, varies according to the refraCtive power of the 
body. 
Independent of the effedt which the particular kind of 
cryftallization of Iceland fpar may have in producing the 
phenomena of double refradtion, it appears very evident, 
that the fadls we have juft ftated point out the exiftence 
of certain attradlive and repulfive forces, which operate 
upon the particles of light, and thus confirm the New¬ 
tonian hypothefis of the materiality and aCtual emiflion 
of light. \ 
Thefe are the remarkable properties which the refearches 
of Malus firft detedled. Their difcovery has opened to 
philofophers a field of the richeft nature, prefenting at 
every ftep new phenomena, which are developed by expe¬ 
riments of the moft entertaining kind, and holding forth 
to inquirers conftant hopes of throwing new lights upon 
a branch of fcience, hitherto confidered as obfcure and 
almoft inaccefiible. But they who rejoice at the profpedt 
thus opened, cannot but deeply lament the hard fate 
which fnatched away, in the prime of his life, the able 
and ingenious perfon to whom fcience has been fo largely 
indebted. See Malus, vol. xiv. p. 233. From what we 
have feen of his writings, it may fafely be pronounced 
that, had his life been prolonged even for a very few years, 
he would have fuccefsfully purfued his inquiries with the 
fame fpirit which firft excited them. His whole proceed¬ 
ings, indeed, manifeft the patience andfagacity that mark 
the true philofopher ; and nothing is more affeCting than 
the unabated ardour with which he continued his invefti- 
gations to the laft; that, even when difabled by ficknefs 
from attending the fittings of the Inftitute, he caufed his 
refearches to be regularly read for him by a friend, till 
very nearly the day of his death. 
It mull next be faid, in juftice to the living, that much 
has been done in the fubjeCt by the exertions of his fol¬ 
lowers : and by none more than thofe of Dr. Brewfter 
and M. Biot. Dr. Brewfter has been moft indefatigable: 
he publiftied, during the courfe of the year 1815, no fewer 
than fix different memoirs on the fubjedlj five in the Phi- 
lofophical Tran factions of London, (for which he was ho¬ 
noured with the Copleyan medal,) and one in the Phi- 
iofophical Tranfadtions of Edinburgh. We (hall firft no¬ 
tice the memoirs of Biot, in tire ivth vol. of the Traite de 
Phyfique ; and then give an account of Dr. Brewfter's 
difcoveries. 
t. M. Biot difcovered that the tourmaline, when very 
thin, refradls doubly, like calcareous fpar; but, when in 
thick plates, it-refradls only lingly. From this it is evi¬ 
dent, that in this mineral there exift two diftindl caufes of 
polarifation; one belonging to the cryftalline molecules 
of the tourmaline, the other depending on the plates of 
which the cryftal is compofed, The firft adts fenfibly only 
when the mineral is very thin ; the fecond, when it has 
a certain degree of thicknefs. M. Biot afcertained like- 
wife, that, when tire agate is very thin, it trahfmits light 
4 
I c s. 
in every direction, and pofieffes the properties of a doubly, 
refradting body. The laws obferved by Dr. Brewfter re- 
fpedting the agate, hold only when it pofieffes a certain 
degree of thicknefs. 
2. After Malus had difcovered the polarifation of light 
when refledted from the furface of diaphanous bodies, he 
examined the metals, and found that polarifation was not 
produced by reflection from them, at lead, in the fame 
manner as from diaphanous bodies. But Dr. Brewfter 
afterwards difcovered that, when a ray of light already 
polarifed is refledted feveral times from the-furface of plates 
of filver or gold, it is modified in fuch a way that, when 
analyzed by means of a prifm of Iceland fpar, it divides 
itfelf into two differently-coloured pencils. Biot, on re¬ 
peating the experiment, obferved that the colours of the 
pencils were precifely the fame with the coloured rings 
obferved by Newton. Thefe obfervations did not coin¬ 
cide with thofe of Dr. Brewfter; but, upon mentioning 
the fubject to M. Arago, that gentleman ftated that he 
had obtained refults fimilar to thofe of Dr. Brewfter, and 
furnifhed Biot with a plate of filver by means of which 
that philofopher was enabled to oblerve fimilar refults. 
Surprifed at this difference, he inveftigated the fubjeCt 
with care, and found that the phenomena depended on 
the way in which the metallic plate had been polifhed. 
There are two ways of polifhing metallic plates ; by ham¬ 
mering and by friCtion. When the former mode is fol¬ 
lowed, the phenomena obferved by Biot are obtained; 
when the latter, the phenomena obferved by Dr. Brewfter. 
Biot at laft afcertained that a metallic furface polifhed by 
friCtion produces two diftinCl effeCts upon light. It gives 
..to a part of the incident light what he calls movable po¬ 
larifation, the fame which is produced by a thin cryftal- 
lifed plate. This occafions the feries of coloured rings of 
Newton. It gives alfo to the white incident light a fixed 
polarifation in the plane of incidence, the fame as is pro¬ 
duced by a thick cryftalline plate. The firft of thefe po- 
larifations is only fenftble in particular pofitions when the 
metallic plate is polifhed by friCtion. Hence the reafon 
■why it was not obferved by Dr. Brewfter; but it is ftrong 
when the plate is polifhed by hammering, and accordingly 
it was obferved by Biot. 
3. Ivl. Biot had fhown long ago, that, when light tra- 
verfes certain cryftals, the repulfive force which produces 
the extraordinary polarifation aCts with more intenfity on 
the violet molecules than on the blue, more on the blue 
than on the green, and fo on, aCting with lead intenfity 
upon the red ray. It is natural to conclude that the ex¬ 
traordinary refraction aCts in the fame manner on the 
molecules of light, iince it is intimately connected with 
polarifation. In a memoir publifhed in the Annales de 
Chimie for June 1815, (vol. xciv. p. 281.) he (bowed that 
this law holds with refpect to Iceland cryftal, and indeed 
all cryftals in general. 
We fliall now proceed to the difcoveries on this fubjeCt 
publiflied by Dr. Brewfter. 
After this gentleman had repeated the experiments of 
Malus, upon reflection it immediately (truck him, that 
light would probably receive a like modification, when 
tranfmitted through bodies wholly or imperfeCtly tranf- 
parent. A numerous courfe of experiments, however, 
appearing to give no new refult, he had almoft abandoned 
the inquiry, when a very fingular appearance of colour, 
in a thin plate of agate, was accidentally prefented to 
him. The plate he employed was about the fifteenth of 
an inch in thicknefs, and was cut by a plane perpendicu- 
cular to the laminae of the agate. The agate was very 
tranfparent, and gave a diltinCt image of any luminous 
objeCt: in addition to this, there appeared on each fide 
of the image another, highly coloured, and forming with 
it an angle of about jo degrees; and, what is moft fingu¬ 
lar, thefe images, when examined with a cryftal of Ice¬ 
land fpar, exhibited all the phenomena of polarifation j 
they alternately van;died and re-appeared as the cryftal 
revolved, juft as if the rays had been tranfmitted, in the 
