Address to the Royal Oeologioal Society of Ireland. Ill 



our Journal a paper, or lecture, communicated to the Society in 

 1836 by Dr. Lloyd, " On the Optical Properties of Crystals and 

 their relation to the Crystalline Forms." In connection with this 

 we cannot refrain from mentioning now, though it has been so 

 often referred to in various books and papers, the delicate and 

 successful investigation by which Dr. Lloyd verified the prediction 

 of Sir William Rowan Hamilton respecting Conical Refraction 

 in biaxial double-refracting crystals. Sir William, from his dis- 

 cussion of Fresnel's theory, perceived that when a ray of light has 

 traversed a biaxial crystal in the direction of either of the lines of 

 single ray-velocity (which lines nearly coincide with the optic 

 axes of the crystal) the linear ray, on emergence, becomes refracted 

 into a cone of rays; this he called external conical refraction. He 

 perceived also that there must be another case of this phenomenon 

 viz., internal conical refraction, which must take place when a ray 

 falls on the crystal so that one of the refracted rays would, ac- 

 cording to the usual law, coincide with an optic axis ; this ray 

 on entering the crystal must be refracted into a cone, which on 

 emergence would become a cylinder. Dr. Lloyd, seeing that 

 arragonite would be specially suitable for the purpose on account 

 of the comparatively large angle between its optic axes, selected 

 that mineral for experiment and satisfactorily proved the correct- 

 ness of both anticipations of Sir W. Hamilton, thereby contributing 

 to a very remarkable triumph of mathematical investigation and 

 corroboration of the undulatory theory of light. 



Another physical subject which was a speciality of Dr. Lloyd's 

 was that of Magnetism. He was an early and prominent investi- 

 gator of the phenomena and laws of terrestrial magnetism, and it 

 was through his influence that the Magnetical Observatory at 

 Trinity College was established in 1838. He was largely instru- 

 mental in prevailing upon the Government in the same year to 

 send an expedition to the Southern hemisphere for magnetic 

 research ; and he and General Sabine were deputed by the Royal 

 Society to go to the continent of Europe to organize magnetical 

 observation there. On this subject he has written "An Account 

 of the Magnetical Observatory at Trinity College, Dublin," 1842, 

 " Dublin Magnetical and Meteorological Observations," 2 vols. 4to., 

 1865-9, " A Treatise on Magnetism General and Terrestrial," 1874, 

 and numerous papers on the subject in the Transactions and 



SciEN. Proc. R.D.S. Vol. hi., Px. hi. K 2 



