C. Travis — Behavior of Crystals in Light. 427 



Art. XXXYIII. — On the Behavior of Crystals in Light 

 Parallel to an Optic Axis ;* by Charles Travis, Ph.D. 



If a section of a biaxial crystal be cut normal to an optic 

 axis, and this section examined in parallel light between 

 crossed nicols, it appears uniformly bright in all positions 

 when rotated about the axis. This is commonly ascribed to 

 interior conical refraction, the explanation given by various 

 authoritiesf being the equivalent of the following : 



When a ray of light, the wave-front of which is normal to 

 the optic axis, enters the section, it is broken up into a cone 

 of rays, each element of which is polarized in a different 

 plane. Hence the light on emerging is polarized in all azi- 

 muths. This is equally true if the entering ray is plane polar- 

 ized, for its vibration will have a component parallel to the 

 vibration direction of each elementary ray of the cone. No 

 matter how the analyzing nicol is placed with respect to the 

 polarizer, then, it will fail to extinguish all the light that 

 comes from the crystal. Following out this line of reasoning, 

 it appears that the intensity of the light passing the upper 

 nicol will be one-half of that from the lower. 



Certain important factors are neglected in reaching this con- 

 clusion, which is untenable when these are considered. It is, 

 therefore, the object of this paper to present a discussion of 

 the behavior of crystals in light that is approximately parallel 

 to an optic axis, and to explain the observed differences between 

 uniaxial and biaxial crystals under these conditions. 



§1. In any pencil of light that it is possible to obtain in 

 practice, there are rays having all directions within certain 

 limits. The energy of those rays that are strictly parallel to a 

 given direction (e. g., the optic axis) is infinitesimal compared 

 to the total energy of the pencil. An example will make this 

 clear. Suppose our source of light is-a circular area of radius, 

 i\ at the focus of a collimating lens of focal length, f. The 

 angular radius of the pencil is then equal to the angle whose 



r 



tangent is-^.. A line drawn through the optical center of the 



lens, parallel to the optic axis, will intersect the source in a 

 point, p, and from this point only do we obtain rays that are 



* This paper was suggested by the work of W. Voigt (referred to below), 

 who shows that interior conical refraction has no practical existence. The 

 writer's chief object is to point out the correct explanation of a phenomenon 

 that is well known to crystallographers. 



f For example, cf. Groth, P., Physikalische Krystallographie, Leipzig, 

 1905, p. 109. 



