OPTICS. 
recovers Its full brilliancy. Hence light, tranfmitted 
otiiiquely through all tranfpare'nt bodies, differs polari¬ 
fation, like one of the pencils formed by doubly-refraCt - 
ing cryltals. By a number of experiments, made with a 
great variety of feries of plates. Dr. Brewfter arrived at 
the law which determines'the relation between the num¬ 
ber of plates and the angles of incidence at which the 
light is polarifed. He found, that “ the number of plates, 
multiplied by the tangent of the angle at which the pencil 
polarifes light, is a conftant quantity.” He alfo difco- 
vered by experiment, that, as the refractive power of the 
plates increafed, the angle of incidence at which the light 
was polarifed diminilhed. This difcovery of the polari¬ 
fation of light by oblique refraCtion, is certainly of much 
importance. It affords an explanation which was want¬ 
ing, to a part of the phenomena which the experiments of 
Mains did not account for. The polarifation of one,of 
the images, formed by a doubly-refraCting cryftal, is ef¬ 
fected by refra&ion, and not by reflection. The difcovery 
of Malus extended only to the polarifation by oblique 
reflection ; but afforded no explanation of the manner 
in which the polarifation of the tranfmitted light was 
produced. 
It has long been known, that fome fpecimens of cal¬ 
careous fpar have the property of multiplying images, 
which are, in almolt all inftances, tinged with very bril¬ 
liant colours. Phiiofophers have generally concurred in 
the opinion, that the multiplication of the images was 
produced by fiffures or fraChir'es within the cryftal, and 
that the colours proceeded from films of air, included in 
c thole (ifl'ures, and were the fame as thofe of thin plates. 
Some experiments which Dr. Brewfter made on thisfub- 
jeCt, led him to believe that fuch opinions were erroneous ; 
that the interrupting plane was not a fraCture; and that 
the colours were analogous to thofe produced by the ac¬ 
tion of cryltals upon polarifed light. According' to his 
view' of the fubjeCt, there is, in every fpecimen of calca¬ 
reous fpar which has the above properties, a plane, which 
he denominates the interrupting plane, lying acrofs the 
cryftal, fo as to divide it into tw'o equal prifms ; and re- 
fembling a thin film, or connecting vein, by which the 
prifms are bound together. This plane is not a ftratum 
of air, as had been conjectured ; for, in that cafe, light 
incident at a certain angle would be wholly reflected from 
the furface of the fecond prifm ; but experiment has de- 
monftratea, that there is no angle of incidence which 
produces a total reflection at the lecond furface. If the 
colours were produced by a fiffure, they would appear 
where the fiflure is bounded by parallel faces; but, if the 
folid angles on each fide of the interrupting plane are 
ground down till turn perfeCtly-fmooth ayid parallel faces 
are produced, it will be found, that, when a ray of light 
is tranfmitted through the plane, there is neither a mul¬ 
tiplication of images, nor the flighted appearance of co¬ 
lour. From thefe and other experiments, which it would 
be tedious to enumerate, Dr. Brewfter concludes, that 
the interrupting plane is a cryftallized vein or ftratum of 
calcareous fpar, cohering firmly to the adjacent mafles ; 
and, that a multiplication of images and colour is pro¬ 
duced only when this vein is interpofed between two 
folid prifms. To confirm this reafoning by experiment, 
he feparated a rhomboid into two prifms ; and interpofed 
between them a thin layer of cement of a different re- 
fraClive power from the calcareous fpar ; and he found, 
that a ray tranfmitted through the mafs emerged in four 
pencils. Hence, the multiplication proceeded from the 
pencil being divided into two by pafiing through the firft 
prifm, and each of thefe two again fubdivided into other 
two by refraCtion at the firft furface of the fecond prifm. 
From the way in which the colours arrange themfelves, 
namely, thole of the middle images being always com¬ 
plementary to thofe of the extreme images, they are ob- 
vioufly produced by polarifed light being tranfmitted 
through the interrupting vein, which in this inftance 
holds the place of the plate of agate in the experiments 
563 
already detailed refpeCting coloured rings. This hypo- 
thefis was alfo verified in a very fatisfaClory manner, by 
placing a thin plate of fulphat of lime between the two 
prifms: when it was cemented in a proper pofition to 
each of the prifms, the fame phenomena were produced 
by this artificial rhomboid as by the natural one. Hence 
the colours depend upon the thicknefs of the ftratum ; 
and. as they are caufed by the feparation of the oppofitely- 
polarifed pencils, there mult neceffarily be a multiplica¬ 
tion of images when colours are produced ; although the 
converfe does by no means neceffarily hold. We may 
further add, that this explanation of the colours pro¬ 
duced by calcareous fpar will account for the appearance 
of colour in veined agate, while fpecimens which are free 
from veins do not exhibit it; for, if we conceive a vein 
to lie between the equi-angular prifms of agate, it will 
exactly correipond to the interrupting ftratum which has 
been affumed in the cafe of calcareous fpar. 
In p. 558. we remarked, that the experiments of Dr. 
Woollafton, and we may add alfo thofe of .Dr. Young, 
had appeared to demonftrate the agreement of the theory 
of Huygens with the phenomena of double refraCtion 
exhibited by calcareous fpar. The inquiries of Malus 
tended ftill further to confirm this agreement. He exa¬ 
mined very carefully quartz, arragonite, and fulphat of 
barytes ; and the refults of his refearches identified their 
optical properties with thofe of calcareous fpar. Hence 
he fuppofed, that the law of Huygens might lately be ex¬ 
tended to all cryftallized bodies. We lhall Ihow, from 
the experiments of Dr. Brewfter, that the Huygenian law 
is not general; that it is not even correCt for the pheno¬ 
mena of calcareous fpar; and that the explanation which 
Laplace attempted of the deviation of the extraordinary 
ray, by referring it to the action of thofe attractive and 
repulfive forces which produce the ordinary refraction 
and reflection of light, as it refts entirely upon the law 
of Huygens, falls to the ground. In the firft place, the 
experiments of Malus upon the three cryltals are erro¬ 
neous; for two out of the three have more than one axis 
of double refraCtion ; confequently, the aberration of the 
extraordinary ray cannot be explained by a Angle ellip- 
foid. Haiiy obferved the property of double refraCtion 
in twenty cryftallized bodies. The patient refearch of 
Dr. Brewfter has detected this phenomenon in no 1 els 
than 165 cryltals. In about a hundred of thefe, he af- 
certained w'hether they had one or more axes ; and, by 
meafuring the tints which they exhibit at different angu¬ 
lar diltances from the axis of double refraCtion, he has 
been conducted to a general principle, to which molt of 
the phenomena of polarifation may be referred. By a 
multitude of experiments, which it is unneceffary to de- 
fcribe, he found that beryl, ruby, potafh, and about 
twenty other cryftals, have one apparent axis of double 
refraCtion, and one apparent axis of polarifation, which 
are coincident; and, whereve'r it was poflible to cut the 
fubltances into tranfparent prifms with large refracting 
angles, he was enabled to afcertain the relation between 
the force of double refraCtion and the polarifing force. 
He found, that the force producing the deviation of the 
extraordinary ray increafes and diminilhes with the po¬ 
larifing force which produces the fylterns. of coloured 
rings. In confidering the nature and properties of thefe 
rings, he had recourfe.to plates of various thickneffes ; and 
the conclulion he atlall arrived at was, that the tints pro¬ 
duced at different inclinations to the axis of the cryftal, 
are to one another as the fquare of the line of the angle 
which the polarifed ray makes with that axis. Hence the 
tint will be a maximum when that angle is 90 0 . 
Dr. Brewfter alfo found, that the compound ftruCfure 
indicative of two or more axes of double refraction, be¬ 
longs to upwards of eighty cryftallized fubltances; and, 
from the lift he has given, it appears, that the combina¬ 
tions of the fulphuric and tartaric acids, with earthy, al¬ 
kaline, and metallic, bales, pofiefs this property more par¬ 
ticularly. There is a funking coincidence between the 
primitive 
