Third Supplement to an Essay on the Theory of Systems of Rays. By WILLIAM 
R. HAMILTON, A. B., M. R. I. A., WR. Ast. Soc. London, M. G. Soc. 
Dublin, Hon. M. Soc. Arts for Scotland, Hon. M. Portsmouth Lit. and Phil. 
Soc., Member of the British Association for the Advancement of Science, Fellow 
of the American Academy of Arts and Sciences, Andrews’ Professor of Astro- 
nomy in the University of Dublin, and Royal Astronomer of Ireland. 
Read January 23, 1832, and October 22, 1832. 
ERRATA. 
Page 60, for Partridge, (Perdix coturniz,) read (Perdix cinerea.) 
Page 67, for pittock, read piltock. 
Page 68, for Insecta, read Crustacea. 
Page 69, for accidentally, read incidentally. 
Of these the theory of external and internal conical refraction, deduced by my 
general methods from the principles of Fresnex, will probably be thought the least 
undeserving of attention. It is right, therefore, to state that this theory had been de- 
duced, and was communicated to a general meeting-of the Royal Irish Academy, not 
at the earlier, but at the later of the two dates prefixed to the present Supplement. 
After making this communication to the Academy, in October, 1832, I requested 
Professor Luoyp to examine the question experimentally, and to try whether he could 
perceive any such phenomena in biaxal crystals, as my theory of conical refraction had 
led me to expect. The experiments of Professor Luoyp, confirming my theoretical 
expectations, have been published by him in the numbers of the London and Edin- 
VOL. XVII, a 
