Minutes of Proceedings. 167 



along a circular line of contact. To each of these geometrical proper- 

 ties an optical phenomenon should correspond, and it "vras an object 

 of great scientific interest to ascertain whether these remarkable 

 deductions from the waye-theory of light Tvould be borne out by- 

 experiment, especially as the optical phenomena they predicted were 

 unlike any known up to that time. The phenomena to be sought for 

 by experiment were — 1°. That a ray reaching the crystal in a direc- 

 tion pointed out by the theory, should be spread out within the 

 crystal along the edges of a cone [internal conical refraction] ; and, 

 2°. That a ray traversing the crystal in another indicated direction 

 should on emergence be expanded into another cone [external conical 

 refraction]. The difficulty of this investigation lay in the precision 

 with which the requisite dii'ections should be given to the rays, and 

 the extreme cai'e with which all other rays, even in closely neigh- 

 bouring directions, should be excluded. Purther, the limits within 

 which the experiments could be made were rendered very narrow by the 

 circumstance that in all known crystals the cone of intersection of the 

 sheets of the wave-surface is very obtuse, and the circle of contact of the 

 tangent plane is a circle subtending a very small angle at the centre 

 of the surface. Professor Lloyd undertook the investigation, and at 

 length, by great patience and wonderful skill, succeeded in making 

 both the experiments, using slices of arragonite, a biaxial crystal. 

 His success in this investigation has furnished us with a verification of 

 the wave- theory of light, all the more significant as the deduction 

 was very remote and the phenomena of a wholly novel character . 



The xvii''' volume of the Transactions of the Academy contains a 

 full account of these researches, which at once lifted Professor Lloyd 

 into the front rank of scientific experimentalists. In 1834 he fur- 

 nished a Eeport to the British Association on " The Progress and 

 Present State of Physical Optics," which is published in the Report 

 of 1834. 



In the field of magnetic research he was equally successful. This 

 was indeed a heritage from his father, under whose auspices the 

 Magnetic Observatory in Trinity College was founded. The Observa- 

 tory was placed under the direction of Professor Lloyd, and was fur- 

 nished with instruments devised by himself, and constructed by an 

 eminent member of the Academy, the late Mr. Grubb. 



Lloyd soon became recognized as one of the leading authorities 



