558 OPT 
it would in all probability have died away entirely, or 
been remembered only to amufe the fpeculations of a 
learned imagination, had not the ingenious experiment- 
alift before mentioned applied it to explain the lingular 
properties of Iceland fpar. By a very Ample apparatus 
which Dr. Woollafton contrived, he was enabled to mea- 
fure, with the utmoft accuracy, the refradtive power of 
the minuted body, whether folid or liquid. He examined 
very attentively the conftrudtion of Iceland fpar; and 
found, that the deviation of the extraordinary from the 
ordinary refradted ray was not a conftant angle, as New¬ 
ton fuppofed : he found that it depended on the pofition 
of the refracted ray; that it was greateft when the ray 
bifedted the obtufe did angle of the cryltal, and lead in 
the tranfverfe direction. In intermediate pofitions, the 
angle of deviation followed fome law which Dr. Wool- 
ladon could not detedt, but for which he obtained a con¬ 
fident explanation, by applying the Huygenian hypothe- 
fis, that the undulations ad’umed the fpheroidal form on 
entering the fpar. Several years afterwards, M. Malus 
made fome very delicate experiments upon this intereding 
fubjedt; and the confirmation they afforded to the firft 
coincidence, gave to that wild theory of vibrations a plau- 
fibility which it certainly did not before appear to poffefs. 
In the fcarcity of optical knowledge, it was natural 
for the fcientific world to hail with joy the difcoveries 
which w'ere publilhed about ten j'ears ago, in the Me- 
moires de la Societe d’Arcueil, by the ingenious French¬ 
man whofe name we have jud mentioned. M. Malus, an 
officer of engineers, who had all his life diown a lingular 
aptitude for fcientific purfuits, retired from the French 
army after the campaign in Egypt; arid, returning to 
Marfeilles, devoted his remaining years to the ftudies 
in which he fo greatly delighted. He was particularly 
defirous of finding the caufe of double refraction; and it 
was in a courfe of experiments inftituted for that purpofe, 
that he made the remarkable difcoveries on which u'e are 
about to enter. “Par des experiences delicates,” fays 
the learned fecretary of the Inditute, “il decouvrait dans 
la lurniere des proprietes remarquables, ou totalement 
inconnues, ou qui n’avaient jamais ete mife en li beau 
jour; enfin-cette reffemblance de la molecule lumineufe 
avec l’aimant, qui fait qu’elle acquiert des poles, et une 
diredtion determinee.” His fird experiments, and his 
reafons for giving the name of polarifation to the pro¬ 
perties he detected, wiil bed be given in his oven words. 
“Let us direct, by means of an heliodat, a folar ray in 
the plane of the meridian in fuch a manner that it makes 
with the plane of the horizon an angle of 19 0 io'; let us then 
fix a plate of glafs, not filvered, in fuch a manner that it 
refledts this ray vertically from above downwards. Then, 
if we place below this fird glafs, and parallel to it, a fe¬ 
cund plate, the laws of incidenceand refledtion will make 
this form with the defeendingray an angle of 35°25'; and 
the ray will be refledted anew in a line parallel to its fird 
diredtion. In this cafe we obferve nothing remarkable : 
but, if we turn the fecond plate, of which the direction 
is now north and fouth, in fuch a manner that its face is 
towards the ead or the wed, without changing the mea- 
fure of its inclination with the vertical ray, it will then 
no longer refledt a Engle molecule of light, either on its 
fird or on its fecond furface; but, if we ltill preferve the 
fame, inclination, and turn the face of the glafs towards 
the fouth, it will again refledf the ordinary proportion of 
the incident light. In the intermediate pofitions the re- 
fiedtion will be more or lefs complete, according as the 
reflected ray approaches more or lefs to the plane of the 
meridian. In thefe circumdances, where the refledted 
ray dilpoles itfelf fo differently, it neverthelefs preferves 
eonflantlythe fame inclination with regard to the inci¬ 
dent ray. Confequently, we fee here a vertical ray of 
light, which, falling on a diaphanous body, difppfes it- 
feif in one manner when the refledting face is turned to¬ 
wards the north or the fouth, and in another when that 
face is turned towards the ead or the wed, although this 
face always forms the fame angle, viz, 35° 25', with the 
I c s. 
vertical diredtion of this defeending ray. Thefe obferva- 
tions lead us to conclude that light, under thefe circum¬ 
dances, acquires properties independent of its diredtion 
with regard to the refledting furface, but only relative to 
the fides of the vertical ray: and which are the fame for 
the north and the fouth fides, and different for the ead 
and the wed. In giving to thefe fides the name of poles, 
I (hall call polarifation that modification which imparts 
to light its properties relative to thefe poles. It is ahnod 
ufelefs to obferve that the plane of the meridian is merely 
chofen for the precifion of ideas, and that the refledted 
ray will undergo exadtly the fame modifications whatever 
may be the cardinal diredtion of the fird refledting furface ; 
that is, the vertical ray will be refledted when the fecond 
furface is parallel to the fird, or when it is in the fame 
plane, but no refledtion will take place when the one 
forms with the other an angle of 90 0 . 
“ I proceed now to a defeription of the phenomena, 
which form the objedt of this memoir. Let 11s confider 
again the apparatus above deferibed. If we prefent to 
the folar ray which lias traverfed the fird furface, and 
of which a part has been refledted, a looking-glafs, fo as 
to refledt from above downwards, we obtain a fecond ver¬ 
tical ray, which will have properties analogous to the fird, 
but in an oppofite diredtion ; for, if we prefent to this 
ray a furface forming with its diredtion an angle of 35 0 
25', and if, without changing this inclination, we turn 
its face fucceflively towards the north, fouth, ead, and 
wed, we may obferve the following phenomena; viz. 
a certain quantity of light will always be refledted by the 
fecond furface, but this quantity will be much lefs when 
the face is turned towards the north or the fouth than 
when it is placed in an ealt or a wed diredtion ; whereas, 
in the fird vertical ray, we obferved exadtly rhe contrary, 
the minimum of refledted light having there occurred 
when the face of the refledting body was in either of the 
latter pofitions. Whence, abdradting from the fecond 
ray that part which dill retains the property of an ordi¬ 
nary ray, and which is equally refledted in all diredtions, 
we fee that this ray retains another portion oflight, which 
is polarifed in a manner exadtly contrary to that of the 
vertical ray refledted by the fird glafs. 
“ Metallic furfaces appeared to me to prefent the phe¬ 
nomena of polarifation in a very incomplete manner. In 
fadt, the ray refledted from them at all incidences is always 
fufceptible of being divided into two pencils by the re¬ 
fledtion of Iceland crydal. We obferve, it is true, when 
the incidence is very great, that one of the images be¬ 
comes faint, W’hile the other increafes in intenfity ; but 
the phenomenon is never fufficiently apparent to enable 
us to determine at what angle it is a maximum. I have 
overcome that difficulty by the following experiment. I 
receive upon a metallic mirror a ray already polarifed, but 
fo that the plane of refledtion, which palies through the 
incident and refledted ray, makes an angle of 4.5° with 
the redtangular poles of the ray. If the incidence is very 
fmall, or very great, the ray is not depolariled. When 
fubjedted to the adtion of a rhomb of Iceland fpar, it is 
always capable of being refradted in a fingle ray; but 
under a mean incidence, and one peculiar to each metal, 
it appears completely depolariled, fo that it may be always 
divided into two rays by Iceland fpar. Hence metallic 
furfaces, as well as other bodies, have a determinate angle 
at which they change the poles of the ray which they re- 
fled!. But, fince in this cafe the plane of refledtion makes 
an equal angle with the two poles, one half of the light 
is polarifed in one diredtion, and the other half in the 
other, fo that the refledted ray alfumes the properties of 
a diredt ray. This experiment fliows us why, if we em¬ 
ploy with metallic mirrors the lame method as with dia¬ 
phanous bodies, the determination of the propofed angle 
becomes impollible. In fadt, when natural light falls 
under the angle propofed, the refledted ray contains at 
once the molecules polariled in one diredtion and in the 
other; fo that, when decompofed by Iceland fpar, it pre- 
fents the fame phenomena as the natural ray, which is 
refledted 
