no 
On the Polarization of Light, 
[April 
Moreover, while the reflected portion is thus situated, with regard to its poles, the 
complementary light has its poles turned to an equal angle on the opposite side of the 
crystalline axis X, or in other words, remains at zero, still in a state fit for trans- 
mission. When the lamina is very thin, the ray which it diverts is always coloured, 
and the colour depends on the thinness, the transmitted ray being always of the 
complementary colour to that reflected. When the lamina is turned round to the azi- 
muth of 45°, the diverted portion has its polar axis at 90°, which, as we have before 
seen, is the fittest for reflection : accordingly, this azimuth is found to give the 
maximum intensity of the coloured reflected ray. One mode of shewing it is by 
laying the lamina horizontally on a dark ground, letting it receive light from white 
clouds, at an angle of 35°, and placing a black glass so as to transmit the rays re- 
flected therefrom : — a portion of the latter will escape transmission, and be re- 
flected by the glass to the spectator, of a brilliant colour due to the dimensions 
of the lamina (figure 23). 
18. The cause ofthis singular division is attributed to the oscillations, more or less 
rapid, which the differently coloured atoms of light experience on entering the sub- 
stance of the crystal (fig. 24), or while subjected to the attractive or repulsive 
energies of its axis. 
If the depth to the lower surface of the lamina (for it is proved that the outer 
surface has nothing to do with the phenomenon) be insufficient to allow the oscilla- 
tion to cease, the atom will reissue with its pole in that attitude in which it had 
just reached the lower surface, and every change of dimension will induce a different 
series of atoms, or colours, ready to issue polarized for reflection , (figure 24.) In 
fact, the experiments of Biot prove that the colours and dimensions of plates pre- 
cisely follow’ the order of the coloured rings observed by Newton, or his “ tits of 
easy transmission and reflection 
Thus, a plate of ,0000151 inch thick, gives a bright blue, by reflection, 
one of ,0000231 yellow' green, 
one of ,0000136 purple, &c. 
The near analogy of these properties, shows the close alliance of all the branches 
of optics ; and suggests the existence, of some general law for the relative actions 
o tlie molecules ot matter and of light, which may at once provide for the cases 
now' distinguished under the several heads of reflection, refraction, dioptrics, chro- 
matics, and polarization. 
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19. The oscillations just mentioned do not commence until the luminous atoms 
have penetrated to a perceptible depth into a crystaliine body, (figure 15 *1 for ex- 
rhnnfl prod “ ce D °™ lour in the redec ^ d or extraordinary image 
of the rbomboidal examiner. The ratio of the thickness of plates corresponding 
to the Newtonian scries of colours, is, as was observed before, the same for all 
substances, but the co-cfficient of absolute thickness differs for each • being 
verselv as t.hp lntpncit-v nf tlw» C-. .• vrtLii , ueixig 
111- 
cr 5?.“'? ■•Active power. Biot gives the following 
table ot these intensities, along with the nature ot the deviative force. 
Carbonate of Lime, .... ****** jXrecfion. 
Sulphate of Barytes 1,25 H 1 1 " " ! ! ! J "" " ? f 7''S 
Sulphate of Strontian, .. 1,07 
Sulphate of Lime, 1,00 
Rock Crystal, 11,00 
Attractive 
Beryl,. 
10 52*} 
tl,e ? ec0 - n< ? 8* ass or rhomboid, (fig. 20,) 
Attractive 
Attractive 
1 
been supposed to be perpendicular to the 
ray. Should the plate niw be m.7e to Tndi^ftomtWs < t* be »*££ 
colour reflected will bp w n incline irom tins position, (figure 31,) the 
fact, the thickness‘of the nlatp J 7 aCc . ordln S t0 the order of Newton’s rings/ In 
P < e goes on increasing as the tangent of the inclination, 
