112 On the Polarization of Light . [April 
rhomboid is turned from right to left. Essential oil of laurel does the same ; but 
camphor, dissolved in alcohol, induces a series from left to right, so that a mixture 
of one part turpentine, and three of camphoric solution neutralize one another. 
Neither water nor alcohol alone exhibit any depolarization on the rhomboid. 
26. Mica presents, under a perpendicular incidence, just the same characters as 
sulphate of lime, and rock crystal plates cut -parallel to the axis ; but it also exhibits 
anomalies of its own, affecting all cases of oblique incidence of the polarized ray. 
These are susceptible of explanation, by allowing to mica two polarizing axes ; one 
in the plane of its laminae, the other at right angles to the first ; both being re- 
pulsive. The yellow imperfect mica is frequently found without the first axis, and 
then, if coupled with sulphate of lime, (which possesses only this axis,) it entirely 
serves to explain the compound nature of the clear mica: and in the same way 
many other combinations may be made and analyzed. 
1 be imperfect mica to which we have alluded, produces no E image in the rhom- 
boid under any azimuth, unless it be inclined obliquely on the ray, with the 
azimuth not in the quadrants. When the plane of incidence is at 45° (the laminae 
being inclined at the same time) the colours of E are most vivid, (fig. 31,) and they 
disappear when the azimuth is brought to 0° or 90°, which proves that the axis of 
polarization is perpendicular to the plane of the crystal. Keeping the azimuth at 
45°, and inclining the mica gradually on the ray, the whole descending series of 
Newton s colours appear. 
The other, or principal polarizing axis of mica, corresponds with one diagonal 
of its primitive rhomboid : its influence modifies the colours visible under oblique 
incidences m a regular manner, increasing up to the angle 35° 11', and thence re- 
trograding; but the cases are too complicated to be embraced in this cursory exa- 
tVif/'ic SU f h b ° dieS nott 1I heir molecules arranged in a definite manner, 
S \ !. e crystallized, as glass, liquids, &c. may be imagined to be com- 
pounded ot numerous regular systems, whose tendency is to neutralize one 
anotuer : if on such bodies some disturbing force be elicited to check their na- 
tural arrangement, ,t may follow that some of these systems (or lamins as 
it were) may remain unneutralized ; and these will then tend to act upon 
a polarized ray in all respects like a crystalline body. Brewster lias paid 
much attention to this species of research, and has shewn that glass, heated red hot, 
and suddenly cooled, (whereby its uniformity of aggregation is prevented ' depo- 
larizes a certain portion of coloured light, depending on the form of the glass. Glass, 
compiessed in a vice, does the same ; and even jelly, when pressed, shews a slight 
powei of depolarization. Borax, melted, and suddenly cooled repeatedly * salt ra- 
&l are su(id« rnnif ,biC ’ evaporated, also depolarize. Horn, bone, ivorv, 
fo™e a n«“o^'^";Sl7„ U erdL need ! “** * regarded in 
de«<i dSSt'”^ by 1 ' e ‘ lection from metallic surfaces was consi- 
tiered doubtful, until Biot and Arago succeeded in rendering it evident bv multiply- 
polished sil^Viloe 
surface is to arrange Us • °i a - mo / ecu ^ e %ht, on approaching a metallic 
of incidence w bil mn , mJf ot po arization parallel to this surface, in the plane 
: “ V i B ot de ' n onstrated, that oscillations of the -• - 
an increased number of reflections*aet^?Ur eye " P1 * W & tin ^y coloured E image : and 
laminae, in changing the order of the ^ ^ mcreased thlckne ss of crystalline 
produces a minor effect of the same nature^ ’ ^ augmented ansrle of incidence 
29 . TI ii — j ’ - • 
90 u. • n j ~ “fiLure. 
Isaac Newton? series ° f coIoured rings, analysed by Sir 
he found from tLcolourofthe ° f ^ntpUtes of mica, &c. may 
Ur °‘ tUc ^Polarized image which they induce, it will be con- 

