New Optical Property of Biaxial Crystals. 511 



traced for a distance of some 30 or 40 cm. from the crystal. 

 The effects are specially beautiful if observed in the light of 

 a mercury vapour lamp. Separate images of the pinhole 

 corresponding to the yellow, green, and violet radiations 

 of the mercury vapour may be seen, and images corre- 

 sponding to the fainter components of the line spectrum 

 may also be observed by cutting out the superfluous light 

 with suitable colour filters. A simple slit with a plate of 

 aragonite crystal and an eye-lens to observe the image thus 

 1 unctions as a little spectroscope, which has quite a marked 

 dispersion in the region of shorter wave-lengths. 



It should be remarked that these optical images formed by 

 an aragonite plate differ from those formed by an ordinary 

 -converging lens in several respects. The images in the 

 present case are real, erect, and of unit magnification irre- 

 spective of the distance of either object or image from the 

 crystal. Further, the image is continuous, that is, it may be 

 observed anywhere in the prolongation of a certain line for 

 a considerable distance from the crystal and not merely at a 

 single point as in the case of the images formed by a lens. 

 Also the images appear sharply denned in a field of diffuse 

 light, showing that only part of the energy passing through 

 the crystal is brought to a focus. The object being fixed, 

 the image moves when the orientation of the crystal is 

 altered, but not when the plate is moved in its own plane. 

 The focussing property, in other words, appears to be related 

 to a fixed direction within the crystal. In order that the 

 image may be within the Held of observation it is necessary, 

 in fact, that the bundle of light-rays should pass through 

 the crystal roughly in this fixed direction, which appears to 

 be that of either axis of single ray velocity in the crystal. 



2. Explanation of the Phenomenon. 



The fact last mentioned suggests a mode of approach to a 

 theoretical explanation of the phenomenon. As already 

 mentioned, the object may be placed at a distance from the 

 crystal, but for simplicity we shall first consider the casein 

 which the object is a point source of light placed just on one 

 of its surfaces. The form of the wave-fronts in the disturb- 

 ance diverging within ihe crystal is the Fresnel surface of two 

 sheets which, as is well known, has a singular point in the 

 direction of either axis of single-ray velocity. If we resolve 

 the wave-front into the gro sp of plane waves of ^hich it is 

 the envelope, we see at once that the singularity is really 

 the crossing point of an infinite number of plane waves the 



