434 C. Travis — Behavior of Crystals in Light. 



In the above discussion, the light in every case is assumed 

 to be monochromatic. When white light is used, we must 

 consider two additional factors. 



(1). The size of the rings in the interference figure in- 

 creases with the wave length of the light ; the average intensity 

 for a pencil concentric about the axis is therefore dependent 

 upon wave length. If the pencil is small, we may say that 

 with light of a greater wave length, less is transmitted. 



(2). In biaxial crystals, the dispersion of the optic axes 

 renders it impossible to center a pencil about the axis for light 

 of more than one wave length. 



The total result of these two factors is that the intensity of 

 the illumination varies with its color, and the crystal in white 

 light, which is approximately parallel to an optic axis, shows 

 impure interference colors. 



Summary. 



In regard to the observed fact that a section of a biaxial 

 crystal, cut normal to an optic axis, is uniformly bright between 

 crossed nicols, we may conclude that : — 



(1). Interior conical refraction, in a strict sense, plays no 

 part whatever as a cause of the phenomenon. 



(2). The cause is to be found in the fact that so-called 

 parallel light has commonly a considerable divergence. 



(3). In any given case, the observed intensity of illumination 

 is equal to the average intensity of that portion of the inter- 

 ference figure bounded by the limits of the pencil of light 

 used. The general configuration of the interference figure is 

 dependent upon the optical constants of the crystal, and upon 

 the thickness of section ; these, as well as the amount of 

 divergence of the light, are the determining factors. 



(4). The reason why the same phenomenon is not commonly 

 observed in uniaxial crystals, is that in the uniaxial figure the 

 first bright ring about the axis is in general much larger than 

 that in the biaxial figure. Under proper conditions, however, 

 the phenomenon may be also shown by a uniaxial crystal. 



University of Pennsylvania, January, 1910. 



