' . 



CONTENTS. 



PAGE. 



I. On the Phenomena presented by Light in its passage along the 



Axes of Biaxal Crystals, 1 



II. Report on the Progress and Present State of Physical Optics, . 19 



III. On a new case of Interference of the Rays of Light, . . .149 



IV. On the Light reflected and transmitted by Thin Plates, . .157 



V. Observations of the Direction and Intensity of the Terrestrial 



Magnetic Force in Ireland, . . . . . . .170 



VI. On the position of the Isogonal Lines in Ireland, deduced from 



the Observations of CAPTAIN SIR JAMES CLARKE Ross, R. N., . 217 



VII. On a New Magnetical Instrument, for the measurement of the 



Inclination, and its Changes, 221 



VIII. Remarks on the theory of the Compound Magnetic Needle, . 231 

 IX. On the Mean Results of Observations, 233 



X. On the determination of the Horizontal Intensity of the Earth's 



Magnetic Force in absolute measure, ..... 246 



XI. On the determination of the Total Intensity of the Earth's Mag- 

 netic Force in absolute measure, by means of the Dip-circle, . 260 



XII. On Earth-currents, and their connexion with the Diurnal Changes 



of the Horizontal Magnetic Needle, 271 



XIII. On Earth-currents, in connexion with Magnetic Disturbances, . 295 



XIV. On the probable causes of the Earth-currents, .... 299 



XV. On the direct Magnetic Influence of a distant Luminary upon the 

 Diurnal Variations of the Magnetic Force at the Earth's sur- 

 face, 30(5 



