OPT 
reflected without polarifation under the greateft and fmall- 
eft incidences, which, in that cafe, renders the limit inde¬ 
terminable. When we fuhjeCt to the reflection of the 
mirror a ray already polarifed, we avoid that inconveni¬ 
ence ; becaufe, inftead of observing, as.with tranfparent 
bodies, theangle under which the polarifation is molt com¬ 
plete, we obferve, on the contrary, that in which the de- 
polarifation is in appearance the molt complete. 
“What diftinguifhes metals from tranfparent bodies is, 
that thefe laft refraCt all the light polarifed in one direction, 
and refleCt all polarifed in the other; while metallic bodies 
reflect what they polarife in both directions. It being 
underitood, however, that they poffefs in part the power 
of all other opaque bodies, of abforbing a greater propor¬ 
tion of that kind of ray which diaphanous bodies tranfmit. 
Thus, for metallic fubftances, we mull employ the reflec¬ 
tion of a ray already polarifed, taking care that the poles 
of the ray form an angle of 45 0 with the plane of inci¬ 
dence, and mult obferve at what angle light appears de- 
polarifed like a natural ray : for diaphanous fubftances, 
on the contrary, we muft employ the reflection of a natural 
ray, and obferve the angle at which the light appears com¬ 
pletely polarifed. The angle, in both cafes, will be deter¬ 
mined .with the fame accuracy. 
“ It is now therefore proved, that all bodies in nature , 
both opaque and diaphanous, polarife light entirely when they 
reflect it under a certain angle; and that the metals, which 
alone appeared to conftitute an exception to this law, on 
the contrary, polarife a greater quantity of light than 
other bodies, fince they refleCt a greater quantity. From 
the preceding experiments there refults a remarkable faCt. 
Place before a luminous body a let of parallel glafs plates, 
and increafe their number till the body ceafes to be leen 
when viewed perpendicularly through them : if you look 
at it obliquely through the fame plates, you will begin to 
fee it again. Underthe angleof incidence 54° 35',its light 
will have the greateft intenfity. Beyond that limit it be¬ 
gins again to difappear. This phenomenon proceeds from 
this circumftance, that the light tranfmitted obliquely 
lofes by polarifation the faculty of being reflected.” 
Mem. of the French Inftitute, 1810. Letter of M. Malus 
to the Foreign Secretary of the Royal Society, 1811. 
The leading phenomena, then, may be briefly ftated 
as follows. When a ray of light falls upon a tranfparent 
body, fo as to be reflected from it, it is modified or af- 
feCted in fuch a manner by this reflection, that, upon 
meeting a fecond tranfparent body, it will either be re¬ 
flected or not, according to the fide which it prefents to 
it. It will be reflected if it fall upon that body on either 
of the oppofite tides, but will not be reflected if it fall 
upon either of the other two at right angles to the former. 
Thus, fuppofe the ray, after being modified by the firft 
tranfparent refleCtor, prefents itfelf to the fecond lo as to 
be reflected; and call the fide of the ray on which it 
meets the fecond reflector the north fide: if the fecond 
refleCtor is turned round, fo that the eafl fide of the ray 
meets it, there will be no reflection; and, in like manner, 
it will be reflected on the fouth, and not on the weft, fides 
refpeCtively. The fame modification, whatever it may be, 
prevents the ray from being doubly refraCted by pafllng 
through Iceland cryftal, which it meets on two of its 
oppofite fides ; but permits it to be doubly refraCted by 
meeting the cryftal on the two other fides. And this 
modification, with refpeCt to double refra&ion, may be im- 
prefled upon the ray by a firft double refraCtion, as well 
as by reflection from a traniparent body. But, where the 
modification is produced by reflection, it is molt complete 
at one particular angle of incidence, which varies in dif¬ 
ferent tranfparent fubftances. 
Now, the exiftence of this phenomenon is certain: it 
is a faCt, that a change takes place in the ray by the ope¬ 
ration of the firft tranfparent body ; it is a fadt that this 
change has fotne kind of reference to the four fides of the 
ray, and affeCts thole fides at right angles to each other 
diiferently. The oblervers of thefe appearances have ex- 
I C S. 55.9 
plained them, by fuppofing that each particle of light has 
its adjacent fides endowed with oppofite properties, and 
that the firft reflecting or doubly-refraCling body turns 
or arranges all the particles of light in a ray, in fuch a 
manner, that their fnnilar fides are prefented in the fame 
direction to the fecond body. Now, this arranging or 
turning of the particles, or this change operated by the 
firft body upon the ray, whatever it may be, is termed, 
from analogy to the phenomena of magnetifm, polarifation. 
Thus, when a ray of light is incident upon a glafsplate 
at an angle of 35 0 25', all the light which the glafs reflects 
is polarifed in one direction. Of the light which is tranf¬ 
mitted, part is polarifed in a direction oppofite to the re¬ 
flected part, and is proportional to that part: the re¬ 
mainder fuffjers no modification, but proceeds as a direCtly- 
tranfmitted ray ; and, if this unmodified part be-received 
by another glafs placed parallel to the firft, and be thus 
decompofed by a fecond refraCtion, the fame phenomena 
are again exhibited ; in faCt, they are rendered more appa¬ 
rent than by the firft arrangement only of the apparatus ; 
for the light which is polarifed by tranfmiflion at each 
paflage, is lefs reflected the oftener it is refraCted ; and 
thus the effeCt in a manner accumulates, and becomes 
more and more perceptible. It alfo appears, that, when 
a polarifed ray is any-how obtained, there is always a cor- 
refponding ray polarifed in a direction diametrically oppo¬ 
fite, and bearing a certain relation to the firft. All dia¬ 
phanous bodies polarife light at certain angles, different 
for each body; and in every inltance there is fome angle 
at which the degree of polarifation is a maximum. 
When a ray of light falls upon a rhomboidal cryftal of 
Iceland fpar, it is lplit in its paflage through the cryftal; 
one part fuffering the ordinary refraCtion, and therefore, 
upon emergence, continuing in the direction of the inci¬ 
dent ray ; while the other undergoes the extraordinary 
refraCtion, by which it deviates by a certain angle from 
the firft. An objeCt viewed through this cryftal will be 
feen under two diftinCt images; and, if the cryftal be turned 
about the axis of villon, the objeCt will frill appear under 
the fame form. Now, let another rhomboid of fpar be 
placed behind the firft, and in a fnnilar pofition, and the 
luminous objeCt from which the ray of light proceeds will 
appear exaCtly as in the firft inftance. Hence the pencil, 
which proceeded in the direction of the ray incident upon 
the firft rhomboid, will continue in that direction ; and 
the pencil which fuftered the extraordinary refraCtion 
will be refraCted by the fecond, but only in the extraor¬ 
dinary manner. But, let the fecond cryftal be made to 
revolve flowly round, the firft remaining fixed, and it will 
appear that each of the pencils begins to feparate into 
two; and, when the cryftal lias turned through an arch 
of 45 0 , the reparation will be complete ; fo that there will 
now be four pencils of rays, and confequently the objeCt 
viewed through the two fpars will appear quadrupled. 
When the fecond cryftal, proceeding in its revolution, 
has completed a quadrant, the four pencils are again re¬ 
duced to two ; but with this remarkable difference, that 
the pencil which fuftered ordinary refraCtion in the firft 
cfyftal now fuffers extraordinary only in the fecond ; while 
that which, in pafiing through the firft, underwent extra¬ 
ordinary refraCtion, now fuftains ordinary only while it 
pafies through the fecond. When the fecond rhomboid 
has revolved luccefiively through arches of 135, 225, and 
315, degrees, each pencil is wholly.fpiit into two, and 
four images are vilible. At the end of half a revolution, 
the pencils are refraCted exactly as in the firft pofition ; and, 
when an arch of 270 degrees has been completed, thefour 
pencils have coliapfed to two, under the fame circu-m- 
liances as the end of the firft quadrant. The luminous 
objeCt has then appeared under the forms of two and four 
images, alternately, as the cryftal has.been turned through 
& h f> h anci °f a revolution ; the pencils which 
have formed thele, and which have been produced by the 
diviliori and lubdivifion of the incident ray, being modi¬ 
fied according to the particular pofition of the two, crys¬ 
tals 1. 
