1830 .] 
On the Polarization of Light. 
in 
and moreover, the refleclive forces of the axis of double refraction, which in the 
first position of the lamina was perpendicular to the ray, now change their direc- 
tion : but if allowance be made for these two influences, the colour and intensity 
of the depolarized ray will be found to be guided by the same law as under the 
perpendicular incidence. 
22. Thin plates of rock crystal, cut perpendicular to the axi3, (figure 28,) have 
also the power of depolarizing a portion of light : that is, they produce a coloured 
extraordinary mage, (which we will in future designate by E, and the ordinary one 
by O,) in the rhomboidal eye-piece : the image, as might be expected, does not 
change when the plate is turned in azimuth, as happened in our foregoing exam- 
ples, because now the axis of the deflecting force remains parallel to the ray in all 
azimuths ; but if the. rhomboidal eye-piece itself be made to perform a revolution 
in azimuth, the whole series of complementary colours will pass in review in each 
quadrant : — those belonging to 0 in the first, becoming after 90° the series of E, 
and vice versa. With some specimens of rock crystal, it is necessary to turn the 
rhomboid from right to left, in others the reverse ; and if two equal plates of this 
contrary disposition be superposed, they entirely neutralize the etfect. ______ 
livhnvm of Intensity 
LcLrriinOe 
' fiery 
of the Extra or divcfry %*y' 
23. Again, taking the first quadrant of revolution, the image E varies conti- 
nually in brightness, and its minimum intensity is found at an angle more and 
more distant from 0°, as the thickness of the plate augments : thus when a lamina is 
0,0156 inch thick, the angle of minimum is 9° 45, and colour blue. 
,0190 11 30 ditto. 
,0402 25 00 indigo. 
,0806 50 00 blue. 
,0120 70 00 ditto. 
,1357 8° 00 indigo. 
When the plate is little less than 0,200, the minimum ceases to be observable*. 
24. The peculiarities thus briefly described, cannot be explained by the theory 
of oscillatory polarization, for by that, E at its minimum should always be colour- 
less, inasmuch as the only effect of interposing the plate of rock crystal, while the 
rhomboidal eye-piece stood at zero, having been to divert a small portion of the blue 
light from O towards creating a faint image, E, O remaining sensibly colourless, the 
effect of turning the rhomboid should be to throw more white rays into E, until at 
45° it should have been equal to O plus , the oi’iginal blue light abstracted. Nothing 
of this kind happens : on the contrary, the extraordinary image first decreases in 
intensity, to a certain angle dependant upon the thickness ot the plate, (fig- 29,) and 
afterwards runs through the descending series of colours, while turning from o to 90°. 
It would involve more space than 1 can spare to explain the theory upon which 
Biot rests this new property: suffice it to say, that he supposes a secondary force 
inherent in the crystal acting perpendicularly to its axis, and tending to make the 
molecules of light rotate on their centres: the violets, being the lightest, will be 
those which will first undergo depolarization. 
25. This rotative force must be independent of crystallization, since several 
liquids exercise the same Turpentine enclosed in a tube, with glass ends, possesses 
B^th of the power of rock crystal, and gives a series of colours to E, when the 
* Some laminae, made up by myself from Delhi crystal, gives nearly similar re- 
sults, viz : thickness 0.032 angle of minimum 10° ^colour violet 
.0.36 20 
.062 36 
.082 J2 
the 3 last together or 0.150 8b 
l a plate of Beryl, 0.045 ° 
blue, direc- 
tion from left 
to right. 
colour white. 
