370 THIRD REPORT— 1833. 



presented itself to M. Cauchy, likewise, in his dynamical re- 

 searches respecting a system of attracting or repelling points. 



On Conical Refraction. By the Rev. H. Lloyd, Professor 

 of Natural and Experimental Philosophy in the University 

 of Dublin. 



Professor Lloyd gave a brief account of the experiments by 

 which he established the existence of conical refraction in 

 biaxal crystals, in conformity with the theoretical anticipations 

 of Professor Hamilton. 



The substance employed in these experiments was arrago- 

 nite, which was selected chiefly on account of the magnitude 

 of its biaxal energy. The specimen was one of remarkable 

 purity, procured by Mr. DoUond. Its thickness was '49 of an 

 inch, and its parallel faces were perpendicular to the line 

 bisecting the optic axes, being cleavage planes of the crystal. 



The first case of conical refraction examined by the author 

 was that called by Professsor Hamilton external conical re- 

 fraction. It was expected to take place when a single ray 

 passes within the crystal in the direction of the line connecting 

 two opposite cusps on the wave-surface. When this is the 

 case, Professor Hamilton has shown that there should be a 

 cone of rays without, the magnitude of which will depend on 

 the biaxal energy of the crystal. In the case of arragonite, the 

 angle of the cone, calculated from the elements of the crystal 

 as determined by M. Rudberg, amounts to 3° very nearly. 



A thin metallic plate, perforated with a minute aperture, 

 was placed on each face of the crystal, and these were adjusted 

 in such a manner that the line connecting the apertures should 

 coincide nearly with one of the optic axes. The flame of a 

 lamp was then brought near the first surface of the crystal, 

 and in such a position that the central part of the beam con- 

 verging to the aperture should have an incidence of between 

 15° and 16°. When the adjustment was completed, there 

 appeared, on looking through the aperture on the second sur- 

 face, a brilliant luminous circle with a small dark space around 

 its centre ; and in this central dark space were two bright 

 points, separated by a well defined dark line. When the plate 

 on the second surface was slightly shifted, so that the line 

 connecting the two apertures no longer coincided accurately 

 with the line joining the cusps on the wave-surface, the phaeno- 

 mena rapidly changed, and ultimately resolved themselves into 

 the two pencils into which a single ray is divided under ordi- 

 nary circumstances. 



