DOUBLE REFRACTION. 153 



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 crystal 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 E 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 Liyht, art. 

 785, et seq. Translator. 



1 * 



