COLOURS OF CRYSTALLINE PLATES. llj-3 



by experiment. When a polarized beam diverging from a lumi- 

 nous origin is transmitted through two rhomboids of Iceland spar 

 of equal thickness, having their principal sections inclined 45 on 

 either side of the plane of primitive polarization, the emergent 

 light will diverge as if from two near points, and the two portions 

 will be oppositely polarized. It was found by Fresnel and Arago 

 that the light resulting from the union of these pencils was plane- 

 polarized, circularly polarized, or e lliptically polarized, according to 

 the difference of the paths traversed when they met. 



Here, then, we have an account of the facts which seem to 

 have suggested the theory of moveable polarization ; and we learu 

 moreover that they are but particular cases of the general result. 

 The light arising from the union of the ordinary and extraordinary 

 pencils, which emerge from the crystalline plate, will be polarized 

 in tl^e primitive plane, or in a plane inclined to it at an angle 

 equal to double the angle which it makes with the principal section, 

 according as the interval of retardation of the two pencils is an 

 even or an odd multiple of half a wave. In all other cases the 

 thickness of the crystal having any other value than those which 

 exactly answer to these intervals the resulting light will be ellip- 

 tically polarized. The ellipse will become a circle, and the .light 

 will appear to be completely depolarized, when the two pencils are 

 of equal intensity, and the interval of retardation is an odd 

 multiple of a quarter of a wave. Here, then, is suggested an easy 

 method of putting the theory of moveable polarization to the test. 

 If a plate of sulphate of lime, whose thickness corresponds to such 

 an interval, be placed in a beam of polarized light of some simple 

 colour, so that its principal section is inclined at an angle of 45 

 to the plane of primitive polarization, the emergent light should, 

 according to the theory of waves, be circularly polarized ; and the 

 two pencils into which it is divided by the analyzing rhomb should 

 not vary in intensity during its revolution. According to tho 

 theory of moveable polarization, on the other hand, the light 

 should be plane-polarized; and one of the images should vanish 

 when the principal section of the rhomb coincided either with the 

 primitive plane, or with the plane perpendicular to it. This experi- 

 tnentum crucis was tried by MM. Fresnel and Arago, and the 

 result was just as had been predicted by the wave-theory.* 



* Anr.ales dc Chimie, torn. \vii. 



