Mr. MacCullagh's Additional Note on Conical Refraction. 197 



measuring about y^th of an inch in diameter; it is usually 

 placed under stones and in crevices of the earth. 



There must be something very remarkable in the internal 

 as well as external organization of this extraordinary spider, 

 for numerous specimens of both sexes submerged in cold water 

 on the 21st of October 1832, remained in that situation till 

 the 22nd of November, an interval of 768 hours, without hav- 

 ing their vital energies suspended. It is evident, therefore, 

 that this species possesses the power of abstracting respirable 

 air from water, for though in the act of submersion the spira- 

 cles are usually enveloped in a bubble of air, yet so small a 

 supply must soon be exhausted, and, indeed, it speedily dis- 

 appears. 



Oakland, near Llanwrst, Denbighshire, 

 July 29, 1833. 



XXXVI. Additional Note on Conical Refraction. 

 %J.MacCullagh, F.T.C.D. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 HP HE introductory part of my note which appeared in your 

 -■- last Number was written in haste, and I have reason 

 to think it may not be rightly understood. You will there- 

 fore allow me to add a few observations that seem to be 

 wanting. 



The principal thing pointed out in the paper that I pub- 

 lished some time ago in the Transactions of the Royal Irish 

 Academy, is a very simple relation between the tangent planes 

 of Fresnel's wave surface and the sections of two reciprocal 

 ellipsoids. Now this relation depends upon the axes of the 

 sections, and therefore naturally suggested to me the pecu- 

 liar cases of circular section in which every diameter is an axis. 

 Thus a new inquiry was opened to my mind. And accord- 

 ingly, without caring just then to obtain final results, which 

 seemed to be an easy matter at any time, I expressed in 

 conversation my intention of returning to the subject of 

 Fresnel's theory, in a supplementary paper. The design was 

 interrupted, and I was prevented from attending to it again, 

 until I was told that Professor Hamilton had discovered cusps 

 and circles of contact on the wave surface. This reminded 

 me of the cases of circular section, and the details given in my 

 last note were immediately deduced. 



I am, Gentlemen, &c. 



J. MacCullagh. 

 Trinity College, Dublin, August 1C, 1833. 



