Hence it is no wonder that the Aurora has always received a considerable amount of attention at the hands of scientific men. Early explorers of the Arctic Regions made constant and important observations of it and its character; and the list of references to works given in the Appendix will show how often it formed the subject of monographs and communications to learned Societies. The early contributions seem relatively more numerous than those of a later date; and the substance of them will be found well summed up in Dr. Brewster’s ‘Edinburgh Encyclopædia’ (1830), article “Aurora.” A most complete and able epitome of our more recent experience and knowledge of the Aurora and its spectrum has been contributed by my friend Mr. Henry R. Procter to the present (9th) edition of the ‘Encyclopædia Britannica,’ article “Aurora Polaris.” It is, however, a drawback to Encyclopædic articles that their matter is of necessity condensed, and that they rarely have the very desirable aid of drawings and engravings to illustrate their subjects. In spite, therefore, of the exhaustive way, both as to fact and theory, in which the contributor to the ‘Encyclopædia Britannica’ has realized his task, it seemed to me there was still room left for a popular treatise, having for its object the description of Auroræ, their characters and spectra. The question of the Aurora spectrum seems the more worthy of extended discussion in that it still remains an unsolved problem. In spite of the observations and researches of Ångström, Lemström, and Vogel abroad, and of Piazzi Smyth, Herschel, Procter, Backhouse, and others at home, the goal is not yet reached; for while the faint and more refrangible lines are but doubtfully referred to air, the bright and sharp red and green lines, which mainly characterize the spectrum, are as yet unassociated with any known analogue. With these views, and to incite to further and closer observations, I have been induced to publish the present volume as a sort of Auroral Guide. For much of the history of the Aurora I am indebted to, and quote from former articles and records, including the two excellent Encyclopædic ones before referred to. Mr. Procter, Mr. Backhouse, and my friend Mr. W. H. Olley have each kindly furnished me with much in the way of information and suggestion. Dr. Schuster has lent me tubes showing the true oxygen spectrum; while Herr Carl Bock, the Norwegian naturalist, has enabled me to reproduce a veritable curiosity, viz. a picture in oil painted by the light of a Lapland Aurora. The experiments detailed in Part III. were suggested by the earlier ones of De la Rive, Varley, and others, and demonstrate the effect of the magnet on electric discharges. For assistance in these I am indebted to my friend Mr. E. Dowlen.