screen, then yellow fhading to red, and laftly crimfon. 
’i he colours in the rings grow fainter as they recede from 
the central fpots. Thofe now enumerated belong to the 
iecond ring ; but, in the fixth ring of the fame fet, the 
colours are only very faint blue, and 11 ill fainter crimfon. 
The part of the firlt ring which is neared to the central 
fpots is always black. If the plate of agate be turned 
round 90°, f'o as to have its laminae parallel to the feCtion 
of the topaz, another fet of elliptical rings will be ob- 
ferved; and the colours in thefe are invariably comple¬ 
mentary to thofe in the firlt fet: the green rings of the 
fecond fet will always be found to correspond with the red 
rings of the firlt let ; the blue with the yellow ; in the 
outer rings, the blue with the pink ; and l'o on. The co¬ 
lours of the central fpots, alfo, in the fecond fet, are com¬ 
plementary with thofe in the central fpots of the firlt; and 
the black fpace which furrounds thefe fpots in the firlt fet, 
is found to be white in the fecond. If a doubly-refraCting 
cryltal be ufed inftead of the agate in one pofition, the 
firlt fet of rings wiil hold the place of the firlt image ; and, 
upon turning the cryltal about its axis through 90 0 , that 
Jet will occupy the Iecond image, and the fecond fet will 
keep the place of the firlt image ; and fo on, alternately, 
through every quadrant. In thefe and fimilar experiments 
upon coloured rings, the Jirfi fet is always the molt dif- 
tinfit of the two; probably becaufethe light reflected from 
the firlt furface is deftroyed by the polariling body : which 
is not the cafe in the pofition of the cryftals that produces 
the fecond fet. 
If, inftead of common light, a beam of polarifed light 
be tranfmitted through the plate of topaz, phenomena 
itiil more varied and interelting are exhibited. When 
light thus obtained by reflection from any tranfparent 
body, lb placed that the plane of its reflection is perpen¬ 
dicular to the plane of the reflection from the topaz, part 
of the rays tranfmitted through the topaz, having been 
already polarifed, will be depolarifed in palling along the 
oblique depolarifing axis of the topaz. If the obferver 
looks into the topaz in the direction in which thefe depo¬ 
larifed rays emerge, he will fee the Jirft Jet of elliptical 
rings coloured, as in the laft experiment; the colours 
being remarkably brilliant and diftinCt. Let thefe depo¬ 
larifed rays be now received upon a plate of agate, placed 
as in the laft experiment, with its lamina: perpendicular 
to a vertical feCtion of the topaz ; and a third fet of rings 
will -be vifible, differing from th z Jirft fet only in having 
frnaller central fpots. This third Jet rather correfponds 
with the fecond Jet in the laft experiment, as their colours 
are complementary, and their central fpots of the fame 
magnitude. Now, let the agate be turned round, fo that 
its laminae are parallel to the plane of the topaz; and a 
fourth Jet of rings will be feen, much lei's brilliant than 
any of the others, and in ftruCture rather refembiing the 
firjl fet. Thefe phenomena are produced alfo when the 
-polarifed light is obtained by a doubly-refraCting cryftal; 
and, as might be expeCted, the third Jet is feen in one po¬ 
fition of the cryftal. occupying the placeof the Jirft image, 
and the fourth in that of the fecond ; and they alternate 
in every quadrant of its revolution. 
Dr. Brewfter mads many other very interelting experi¬ 
ments upon this part of the fubject. What we have 
analyfed, is fufficient for the purpofe of illuftrating thefe 
amufing phenomena ; and it would be ufelefs, even if 
our limits would permit us, to enumerate more. With 
refpeCt to any law to which thefe rings can be referred, he 
informs us, that, by aCtual meafurement of the angles 
which the colours fubtend, ufing a plate of topaz of one 
thicknefs, compared with the angles they fubtend when 
topaz'of a different thicknefs is employed, he has afeer- 
tained, that “ the conjugate diameters of the rings are in¬ 
versely as the thicknefs of the piates.” Rock-cryftal, eme¬ 
rald, amber, nitrat of potafli, and many other lubftances, 
produce elliptical coloured rings in as great perfection 
as the topaz 
Wefhave already mentioned fonie bodies which have 
the property of depolarifing light, in whatever pofition 
they are held. Of this fort are gum-arabic, caoutchouc, 
and feveral others, which are all formed by a fuccefllve 
depofition and induration of layers. The nature of their 
formation very fatisfaCtorily explains the reafon of this 
peculiar character. Let us conceive, that, in a piece of 
gum-arabic, Tor example, the firlt layer only is depofited 
and cryftallized. It will obvioully poffefs both neutral 
and depolarifing axes, like any other cryftal. In like 
. manner, the fecond layer will alfo have thefe axes; but 
when it isdepofited upon the firlt, there is no reafon why 
their axes Ihould coincide; on the contrary, they will 
probably hold very different directions. Thus, after a 
great number of fuccefiive layers, there will be different 
depolarifing axes lying in every different direction. Now, 
it is found, that, if one plate of mica be placed upoy an¬ 
other, fo that the depolarifing axis of the one coincides 
with the neutral axisof the other, all the axes will affume 
the character of depolarifing axes ; and the two plates 
will aCV upon light exaCtly like gum-arabic. Hence, lince 
the depolarifing axis, coinciding with the neutral, produces 
a depolarifing axis, we may conclude, that, in a piece of 
gum-arabic, or caoutchouc, confining of a great many 
layers, ail the axes will be depolarifing axes. This expla¬ 
nation is likewife verified by taking a very fine film of 
either of thefe fubftances; it will be found to have neutral 
axes, and "to have loft the power which it before had, of 
depolarifing light in every pofition. 
We'fiiall only mention one more of the curious refults - 
which Dr. Brewfter obtained, in his experiments on the 
depoiarifation of light, namely, a remarkable property 
which is produced by Ample preffure, in foft tranfparent 
foiids, by which they acquire all the charaders of cryftals 
that both polarife and depolarifie. The fubftance which 
Dr. Brewfter operated upon was calves’-feet jelly. A 
folid portion of this, about half an inch in thicknefs, was 
placed between two plates of glafs, and was obferved to 
have no power whatever in depolarifing light. When it 
had remained borne days in this fituation, it began to de- 
polarile about its edges ; and, in a fortnight, this property 
was extended over the whole of its l’urface. In three 
weeks, the jeliy began to lofe its tranfparency, and alfo 
its depolarifing quality : it was now much indurated, and 
reduced to about one-fourth oi its original bulk; in tena¬ 
city, it very much refembled caoutchouc. Upon inter- 
poling it between the light of a taper polarifed by reflec¬ 
tion, and a prifm of Iceland fpar, it appeared to produce 
no effect ; one of the images of the paper regularly va- 
niftiing as the fpar was turned round 90 0 . While the fpar 
was in (uch a fituation that only one image of the taper 
was feen, Dr. Brewfter preffed together the two plates of 
glafs, and was altonifhed to find the vanifhed image in- 
itantly become vifible. Hence, the jelly had, by preffure 
alone, acquired the power of depolarifing the light, to 
which this image owed its exiftence. When the preffure 
was removed, the image again difappeared, the jelly being, 
by its elafticity, reftored to its original ftate. 
While Dr. Brewfter was engaged in feme experiments 
upon the oblique depolarifing axis of mica, he oblerved, 
in a certain pofition of the mica, home indication of a po- 
larifation of the incident rays; arid, upon turning the 
mica round, the quantity of polarifed light increafed as 
its obliquity to the incident pencil increafed. When a 
plate of glafs was fubftituted lor the mica, the fame phe¬ 
nomena were vifible, but in a lefs diltinCt degree ; but 
they became more perceptible upon increafing the num¬ 
ber of the glafs plates ; and, when the plates amounted to 
fifteen, the tranfmitted pencil was wholly polarifed at an 
angle of about 70 0 . The light, thus modified, has all the 
characters of light polarifed by reflection, or by any other 
means. For inftance, if a beam polarifed in this way is 
viewed through a plate of agate, having its lamina: pa¬ 
rallel to the plane of reflection, the bright image vanifhes, 
and the nebulous light which furrounded it attains its 
greateft luftre ; and, when the agate is turned round, fo 
that its laminae are perpendicular to the plane of refrac¬ 
tion, the nebulous light dilappears, and the bright image 
recovers 
