OPTICS. 
flrft inftapCe, through a prifm of doubly- re fra ft i tig fpar. 
“When the common colourlefs image was viewed in the 
lame manner, the like appearances were feen. When the 
flame of a taper, reflected as in the experiments of Mains 
before defcribed, is viewed through the plate of agate, a 
remarkable appearance is obferved. If the laminae of 
which the agate is compofed are parallel to the plane of 
refleftion, the image of the taper appears perfectly dil- 
tinft; but, when the plate is turned round, fo that its 
laminae become perpendicular to the plane of reflection, 
the light which forms the image of the candle is entirely 
rtfl&led, and not a Angle ray is tranfmitted, or penetrates 
the agate. A fimilar character is obferved when an ob¬ 
ject is feen through two plates of agate : While they are 
held in fuch a pofition that their laminae are parallel to 
each other, the image is feen plainly ; but, if one of the 
plates is turned about till its laminae are perpendicular 
to thofe of the other, then the light is wholly reflected, and 
the luminous objeCt is no longer vifible. There was an¬ 
other appearance obferved, of confiderable importance, as 
it feems to throw more light upon the phenomena of 
double refraftion than any experiment yet mentioned. 
The bright image which was viewed through the plate of 
agate appeared furrounded by a large maf's of nebulous 
light, extending about 7 0 30' in length, and i° 7' in 
breadth, on each fide of the image. Upon interpofing a 
prifm of Iceland fpar between the agate and the eye, and 
turning it flowly round, the nebulous light grew brighter 
and brighter as the bright image vaniflied, and became 
almoll imperceptible when the bright image was feen in 
its fulleft luftre. The nebulous light, then, and the 
bright image, feerned to undergo the fame modification as 
two images formed by a doubly-refrafting fpar ; and Dr. 
Brewfter from this conjeCtured, that the nebulous light 
was in fnCt caufed by the fame ftrufture in the refrafting 
medium, which, in the cafe of Icelandic fpar, produces 
two diftinft images ; in other words, that the agate was 
an approach to that particular kind of cryftailization, and 
that the hazinefs was an imperfeCt image arifing from that 
imperfeftion off ftrufture. It has always been fuppofed, 
that, in a doubly-refraCling medium, the two images were 
produced by two different refraftive powers ; reafoning 
then, from analogy, the bright image and the nebulous 
light produced by the agate could be feparated, the one 
from the other, by making the agate«into a prifm ; and 
the reparation would be proportioned to the angle formed 
by the refrafting planes of the prifm. Dr. Brewfter, ac¬ 
cordingly, tried this experiment, but without fuccefs. 
For fome time he abandoned the inquiry, and began to 
look upon the phenomenon as one of thofe unaccount¬ 
able appearances which fo often attend the paffage of 
light through diaphanous media. Future experiments 
have, we think, very fatisfaftorily. eftablifh'ed the raft, 
that this nebulous light is an approach to afecond image ; 
they depend, however, upon fome appearances which we 
have not yet mentioned, and which we mult firft proceed 
to confider, before concluding our remarks upon this 
part of the lubjeft. 
When a tranfparent fubftance, fuch as rock-cryftal, is 
held in a certain pofition before a ray of polarifed light, 
the light becomes depolarifed, or converted into common 
light. This remarkable property is common to almoft all 
tranfparent bodies when they are held in certain pofitions; 
and there are fome, as gum-arabic, horn, glue, and tor- 
toife-thell, which depolarife light, in whatfoever pofition 
they are held. Among the fubftances which pofiefs this 
property, may be noticed more particularly mica and 
topaz, as exhibiting fome curious phenomena. If a fquare 
plate of mica be fet in a vertical pofition, and a cryftal of 
Iceland fpar be placed either horizontally or vertically 
with refpeft to it, as it were upon a line bifefting any 
two oppofite fides of the fquare, and a ray of polarifed 
light be viewed through both, no change whatever ap¬ 
pears to take place upon the ray. Thefe-lines may, there¬ 
fore, be called the neutral axes f the mica. Now, let the 
Vol, XVII. No. 1197. 
561 
cryftal of Iceland fpar be placed upon either of the. lines 
bifefting two oppoiite angles of the fquares ; and the po¬ 
larifed light will then be feen converted into common 
light, that is, it will be depolarifed. Thefe lines are pro¬ 
perly called the depolarijing axes; and thefe axes (the two 
neutral in a horizontal and vertical direction, and the 
two depolarifing in a direftion bifefting the right angles 
formed by the neutral) are common to all fubftances 
which have the power of depolarifing light. There is 
another remarkable property which attends plates of 
mica, and many other cryftallized bodies, in addition to 
thofe juft mentioned. Let a polarifed ray be obferved 
through the Iceland fpar and mica: when the fpar is 
placed upon the vertical neutral axis of the mica, as we 
have juft feen, the ray Hill continues polarifed ; but, let 
the plate of mica be inclined forwards at an angle of 45 0 
to the horizon, and the image, that was before in vifible 
from polarifation, will now diltinftly appear; and, confe- 
quently, the light to which it owes its exiftence has be¬ 
come depolarifed. The mica then poffeffes an oblique de- 
polarijing axis; and only this one : for a fimilar experi¬ 
ment, made by adjufting the fpar upon the horizontal 
neutral axis, produces no Inch effeft. 
We now recur to the nebulous light. When a pencil of 
light, firft polarifed and afterwards depolarifed, is tranf¬ 
mitted through a plate of agate, the red rays go to the 
formation of the bright image ; while the green, which 
are complementary to the red, compofe the nebulous 
light by which the bright image is furrounded. The ap¬ 
pearance thus prefented, is that of a red image enveloped 
in a cloud of green light. Let the agate be turned through 
90 0 , the colours are reverfed, and the appearance now ex¬ 
hibited is that of a green image enclofed in a cloud of red 
light. In the common experiment, when a doubly-re- 
frafting cryftal is ufed inftead of the plate of agate, it will 
be found that the ordinary image is green, while the ex¬ 
traordinary one is red ; and, while one cryftal revolves, 
an alternation of thefe colours will be obferved in every 
quadrant of its circular motion. We may hence very fa- 
tisfaftorily conclude, that, as the light in this inftance is 
known to be polarifed, fo that which is viewed through 
the plate of agate, fince it exhibits exaftly fimilar pheno¬ 
mena, has undergone a like modification; and,_as the 
nebulous light appears thus to have the fame relation to 
the bright image which the image formed by the extra¬ 
ordinary has to that produced by the ordinary refraftion 
in doubly-refrafting cryftals, we may confider the agate 
as poflefling, imperfefily, the property of double refrac¬ 
tion, (in oppofition, certainly, to the analogy of all dou¬ 
bly-re fradting cryftals, that the two images are produced 
by different refraftive powers;) but that it gives two 
images, and polarifes them like other cryftallized bodies, 
only that the one image is placed in the centre of the other. 
Before leaving the fubjeft of depolarifation, we muft 
mention fome curious relults which Dr. Brewfter was led 
to, in a l'ubfequent inquiry, but which are fo connefted 
with the preceding ftatements, that vye cannot conve- 
niently omit them in this place. We have feen that 
light, which is polarifed by one fubftance, can be depo> 
larifed by another. Thus, two fubftances were necefiary to 
deprive light of its polarity. But, in all bodies which 
have the oblique depolarifing axis, light may be polarifed 
and depolarifed by the fame fubftance ; and this is the 
cafe, when the pofition of the depolarifing body is fuch, 
that the angle which the oblique depolarifing axis makes 
with the under furface of the body, is.nearly equal to the 
complement of the angle at which the incident ray is po¬ 
larifed by its firft refleftion. If a plate of topaz be held 
in this.pofition, and the obferver look into the topaz in 
the direction of emergence of the depolarifed ray, through 
a plate of agate having its laminae perpendicular to a ver¬ 
tical feftion of the topaz, he will perceive about teivbril- 
liantly-coloured elliptical rings, formed round two oval 
central fpots, compofed of different colours. Each rino- 
alfo contains feveral colours, fuch as blue Ihading off to 
7 D green, 
