Rev. II. Lloyd on certain Phenomena of Light. 207 



ring, or as one of the straight pieces in fig. 9, permitting it to 

 advance towards his front ; his left hand will be the unerring 

 index to point out the direction of the passing electric cur- 

 rent. 



A walking-stick, or any other such article, may very well 

 represent the metal to be excited; then a person standing in 

 the position of the polar magnetic lines, as represented in fig. 

 7, 8, and 9, and holding the stick before him, by its extre- 

 mities, one in each hand, and at right angles to the axis of his 

 person, or to a straight line drawn from his head to his feet, 

 will, by pulling the stick towards him, show the proper direc- 

 tion of motion for effecting the greatest degree of excitation 

 under the conditions laid down in Position 4 ; and by the il- 

 lustration of Position 7- the current would flow through the 

 stick from the right to the left hand. 



The preceding positions will, if I have not deceived myself, 

 exhibit a correct view of one class at least of the natural ele- 

 ments of magnetic electricity ; viz. those secondary theoretical 

 laws which govern its excitation, and give direction to its polar 

 streams. They are those 'proximate laws by which the display 

 of the phaenomena is accomplished and regulated, and by 

 which it may very simply be explained, and easily under- 

 stood. By these laws the experimenter may be directed in 

 his manipulation, and with precision he may foretell the direc- 

 tion of the resulting electric streams. 

 [To be continued.] 



— 



XXXIII. Further Experiments on the Phenomena presented 

 by Light in its Passage along the. Axes of Biaxal Crystals. 

 By the Rev. Humphrey Lloyd, A.M. M.R.I. A. Felloxv of 

 Trinity College, and Professor of Natural and Experimental 

 Philosophy in the University of Dublin* '. 

 \ STATED in a former communication-}-, that by a new de- 

 * velopment of the undulatory theory of light, in its appli- 

 cation to the laws of double refraction, Professor Hamilton 

 had arrived at the remarkable conclusion, that in two cases of 

 refraction in biaxal crystals, a single incident ray ought to be 

 divided into an infinite number of rays, constituting a refracted 

 cone. The first of these cases of conical refraction will take 

 place at the emergence of the ray into air, when it has pro- 

 ceeded from a point on the surface of, or within, the crystal, 

 and in the direction of the line:}: joining two opposite cusps in 



* Communicated by the Author. t Page 112, et seq. 



X It is much to be desired that these lines, — the normal to the circular 

 section of the surface of elasticity, and the normal to the circular section of 

 the ellipsoid of FresnePs theory, — were distinguished by some appropriate 



