DOUBLE REFRACTION. — 
If we call to mind that rays of light are so immensely 
attenuated that myriads of them can pass through the 
eye of a needle without mutual disturbance, reflecting 
minds will recognize how much there is-most admirable 
and almost incomprehensible in the fact which we have 
just cited, the discovery of which is also due to Huy- 
ghens. The two pencils of rays which, after emergence 
from the crystal of Iceland spar, have sides endued with 
different properties, are called rays “polarized” in con- 
tradistinction to rays of natural light, possessing the same 
property all round their circumference, since they sepa- 
rate into two beams of the same intensity in whichever 
direction their sides may lie with respect to the form of 
the crystal with which they are analyzed. I have men- 
tioned what ought to be the position of a second crystal, 
so that the ordinary and extraordinary rays emerging 
from the first erystal may preserve respectively the same 
denominations. In the intermediate positions of the sec- 
ond crystal, the rays, whether ordinary or extraordinary, 
coming from the first, in general divide themselves each 
into two, but the intensities of the two portions are ordi- 
narily very different. 
changes in magnitude in these resolved parts will give the relative 
brightness of the images. 
Rays whose sections are represented as in the figure, are said to be 
polarized in the planes of o and & respectively; but it was long a dis- 
puted question whether the vibrations of which they consist, according 
to the wave theory, are actually performed in those planes, or perpen- 
dicular to them; the latter has now been shown to be the fact. 
It need hardly be added that this can be considered only as a very 
general and popular kind of illustration; and for the more exact state- 
ment cf the laws of these changes, especially with regard to the rela- 
tive distances of the several images, or differences of ordinary and 
extraordinary refraction, recourse must be had to more profound 
mathematical investigations. See especially Herschel on Light, art. 
785, et seg.— Translator. 
7* 
