Sir William Rowan Hamilton 137 



led to an optical discovery that created considerable interest 

 at the time because it drew attention to a curious phenomenon 

 of refraction in biaxal crystals which had not previously 

 been noticed. According to Fresnel's theory, there are in 

 such crystals two directions such that a ray passing along 

 them will emerge as a conical pencil. It follows that, under 

 certain experimental conditions, the two spots of light 

 produced by double refraction are spread out and joined so 

 as to form a ring. Hamilton's prediction was immediately 

 verified by Humphrey Lloyd, and was received as a striking 

 confirmation of Fresnel's theory. 



The later years of Hamilton's life were spent in developing 

 the new calculus of " Quaternions," to which he attached 

 great importance; but, though it has yielded methods of 

 great elegance, it has not quite fulfilled its early promise, 

 and has few adherents at the present time. Some of its 

 conceptions, however, permanently survive in the modern 

 vector analysis. 



No single teaching institution has a higher record of 

 scientific output during the last century than Trinity College, 

 Dublin. Humphrey Lloyd, James McCullagh, John Hewitt 

 Jellett, George Salmon, Samuel Haughton, George Francis 

 Fitzgerald, Charles Jasper Joly are names that any University 

 would have reason to be proud of. Lloyd (1800-1881) has 

 already been mentioned in connexion with the verification 

 of conical refraction. In later years he devoted much time 

 to the study of terrestrial magnetism, and took an active 

 part in the magnetic survey of Ireland. James McCullagh 

 was an eminent mathematician whose contributions to the 

 undulatory theory of light take a conspicuous place in the 

 history of that subject. Jellett (1817-1888), like McCullagh, 

 was a mathematician, primarily attracted more by physical 

 and even chemical problems than by pure theory. He is, 

 perhaps, best known for his improvement of the experimental 

 methods for studying the rotation of the plane of polarization, 

 observed in certain bodies like sugar. George Salmon (1819- 

 1904), for many years Provost of Trinity College, confined 

 himself to problems of Pure Mathematics, notably in the 

 domain of Geometry. Samuel Haughton (1821-1897) was 

 primarily a geologist, but his versatile mind made frequent 



