Light Science for Leisure Hours

na which the skies present to our view. There is something surpassingly beautiful in the appearance of the true 'auroral curtain.' Fringed with coloured streamers, it waves to and f

turbance traverse the magic2 canopy. Startling coruscations add splendour to the scene, while the noble span of the auroral arch, from which the waving curtain seems to depend, gives a grandeur to the spectacle which no words can adequately describe. Gradually, however, the celestial fires which have illuminated the gorge

ikingly beautiful auroral displays may sometimes be witnessed. Yet those who have seen the spectacle presented near the true home of the aurora, recog

more even than terrestrial, significance. They learned to associate it with relatio

ed the phenomenon as a local display, they could3 form no adequate conception of its importance. The circumsta

watched its course as it began to retrace its way towards the true north. He discovered the minute vibration which the needle makes each day across its mean position. He noticed that this vibration is variable in

ral streamers; but we now reach the important disc

s, the magnetic needle is often seen to move in a few instants over several degrees.' 'During an aurora,' he adds, 'one often sees in the northern region of the heavens luminous streamers of different colours shoot from all points of the horizon. The point in the sky to which these streamers converge is precisely the point to which a magnetised needle suspended by its centre of gravity directs itself.... It has, moreover, been shown that the concentric circular s

f which appeared at first sight of great force. Thus, he remarked that magnetic disturbances of the most remarkable character have often been observed when no

to auroras observable in the immediate neighbourhood, but to auroras in progress hundreds or even thousands of miles away. Nay, as inquiry progressed, it was dis

reamers than even Arago had supposed. It is not merely the case that while an auroral display is in progress the needle is subject to unusual disturbance, but the movements of the needle are actually synchronous with the waving movements of the mysteri

hole earth must be simultaneously disturbed; and secondly, the auroral streamers which wave across the skies of one country must move s

ances which give interest and significance to the s

the sun. The needle in its daily vibration follows the sun, not indeed through a complete revolution, but as far as the influence of other forces will permit. This has been abundantly confirmed, and is a fact of extreme importance in the theory of terrestrial magnetism. Wherever the sun may be, either on the visible heavens or on that half of the celestial sphere which is a

on of these disturbances, in a period of about eleven years, should be found to correspond

days and seasons, so they are subject to the same mysterious influence which causes the northern banners to wave resplendently over the star-lit depths of heaven. Nay, it is even probable that every flicker and coruscation of our auroral displays corresponds with similar manifestations upon every planet which travels round the sun. It becomes, then, a question of exceeding interest to inquire what is the nature of the mysterious apparition which from time t

disturbance, would at once enable us to reject some of these hypotheses. But we need not discuss the subject from this point of view, because a mode of research has recently been rendered

converts its light into a rainbow-coloured streak. If the object is a luminous vapour, its light is converted into a few bright lines. And lastly, if the object is a luminous solid or liquid shining t

which render the interpretation of a spectrum

point, because we are absolutely certain that the sun is self-luminous. Again, we observe the spectrum of the moon to be exactly similar to the solar spectrum, only, of course, much less brilliant. And here also we feel no doubt in interpreting the result. We know, certainly, that the moon is not self-luminous, and therefore we conclude with the utmost certainty that the light we receive from her is simply reflected solar light. So far all is clear. But now take the case of an object like a comet, which may or may not be self-luminous. If we find that a comet's spe

as passed, but of the substances between which the10 spark has travelled. Thus, if we cause an electric flash to pass between iron points through common air, we see

as analysed by the spectroscope, because there are many difficulties in forming a general opinion as to the nature

reader will presently see why I have brought ?ngstr?m's name prominently forward in connection with the interesting branch of spectroscopic analysis just referred to. If the discovery we are ap

by prismatic analysis whether the light of the aurora is or is not electric.' Singularly enough, however, the first proof that the auroral light is of an electric nature was derived from a very different mode of inquiry. Dr. Robinson, of Armagh, discovered in 1858 (a year before Kirchhoff's recognition of the powers of spectroscopic analysis) that the light of the aurora possesses in a peculiar d

bright line only was seen! Otto Struve, an eminent Russian astronomer, shortly afterwards made confirmatory observations. At the meeting of the Royal Astronomical Society in June, 1868, Mr. Huggins thus described Struve's results:12-'In a letter, M. Otto Struve has informed me that he has had two good opportunities of observing the spectrum of the aurora borealis. The spectrum consists of one

conclude, with every appearance of probability, that the luminosity of the vapour is due to the passage of electric discharges through it. It i

ve lines so closely approximating in position that only the most powerful spectroscope can indicate the difference. So that when ?ngstr?m remarks that the bright line he has seen lies slightly to the left of a well-known group of lines be13longing to the metal calcium (the principal ingredient of common chalk), we are by no means to infer that he supposes the substance which causes the presence of t

ed the same bright lines as the aurora. But recent observations show that the coincidence is not so

zine, February 1870