On the Polarization of Light 
Iceland Jsftai 
lauinnaline. €r/std / 
transmit the polarized rays t>f one denomination : this effect holds only when the 
ray under examination is polarized, and when the perpendicular axis of the tour- 
maline is situated in the plane of polarization. We have, therefore, through its 
means, a mode of determining both the position of the polarizing axis of any other 
crystal, and the nature of the polarization, whether positive or negative. Place the 
tourmaline over the image to be examined, and turn it in azimuth, until the image 
vanishes ; then if the side of the prism is parallel to the axis of polarization, the 
nature of the latter is positive or ordinary. 
11. The two images produced by a clear rhomboid of Iceland or calcareous 
spar, are sensibly equal in intensity and colourless. A line drawn through the two, 
is always parallel to the “ principal section” or “ short diagonal” of the rhomboid, 
(fig. 15). When the crystal is made to revolve, one image seems moveable : this 
last is the ordinary ray. Now, by analyzing the two images by the tourmaline 
plate, the ordinary ray, O, is found to Lave its polar axis parallel to the diagonal, 
and the extraordinary one, E, perpendicular thereto. The original natural ray, 
therefore, by traversing the crystal, has been precisely affected in the same way as 
when it met the surface of a glass plane, at an angle of 35° and in every in- 
stance one may be substituted for the other 
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When the rhomboid (or what is better, a prism of the spar, achromatized by a 
pri&m of glass) is used instead of the second reflector of our instrument then with 
the short diagonal parallel to the plane of reflection, we shall have but one image 
transmitted, (the ordinary or positive.) Turning the crystal round to 90 and 270°, 
* at r,g . ht angles to the first reflecting plane, the polarized ray of light again tra- 
visfbK ir T extraordinar y dnage. In al/other aziSs, twoF 
iKllIof devdattn’ luminous intensity will vary as the sine and cosine of 
ted ray in figure 4 ’ J W&S Cd l ° bappen with the reflected and transmit- 
dot in fiem^nf Xn°i^ b % PlaCed *-° Ver an ° ther ’ and aa ima ^ be viewed (as the 
revolve - for toe are nn! ° U " n makin * Supper rhomboid to 
the two \l: g £Z™re to ^£*£22 * be , acted on. At 0- 
thickness : at 180° thev wilVbe brought *5?’ J* tbe < ! r F stal were of double 
boids he of the JSfcSL&S? ^ 
have vanished, but two new ones thZ i V *7 or }S^al images will 
place: at 45 °, 135 ° & c s 0) , r reci P r ocaIs of each, will have taken their 
visiWe - two of w “ 
same; but in a prism ofsilex or rock cmi.? !?“ V ’ at right an £ les to tlie 
to the double refraction of the former beinJ* ’ lt / SJUSt reverse. This is ascribed 
The extraordinary ray, pvopevl^o fc f ^ ^ attractive, fig. 18, 
same species of polarization Te • d ? 0t bot ‘ bears ’ therefore, real ly, the 
POiauzation, e . perpendicular to the Q f crystallization, and 
