i88 



NATURE 



\7an. 6, 1876 



The Glow-worm 



Although in several Natural History Encyclopedias Scot- 

 land is excluded from the list of countries containing the glow- 

 worm, I can aver that in Nithsdale and in the parish of Tynron, 

 Dumfriesshire, they are quite plentiful. Yestreen, in Tynron, I 

 observed one, to my surprise, shining by the wayside. It is a 

 proof of the mildness of the season, no doubt, as I never saw 

 them in December before, but have seen them several times as 

 late as October. 



Wiien carrying one home one evening in my open hand it 

 contracted itself and leaped out of my hand. This is a power 

 they possess which I have seldom seen mentioned. The light 

 in winter is much feebler than in summer, but the time was 

 ten o'clock, or more than six hours after sunset that I saw it, 

 whereas I never witnessed the glow of one in summer so long 

 after dusk. Some that died with me forcibly reminded me of 

 the poet's remark that between the rose's shadow and the very 

 rose there was not a greater contrast than that between ' ' the 

 dead glow-worm and the worm that glows." J. Shaw 



Tynron Schoolhouse, Dec. 26, 1875 



OUR ASTRONOMICAL COLUMN 



Encke's Comet. — By the calculations of Encke and 

 others vs^ho have continued them, we are in possession of 

 the dates of perihelion passage of the comet which bears 

 his name, from 1786 to 1875. If these be arranged and 

 the intervals taken between the successive dates, it will be 

 found that in the course of these ninety years the effect of 

 perturbation has not changed the period of two succes- 

 sive revolutions by a hundredth part. The revolution 

 18 19- 1 822 was lo'i days longer than that between 1815 

 and 1 8 19, and the revolution 1845- 1848 was in days 

 shorter than the preceding one, and these are the largest 

 variations exhibited. In the same period, the longest 

 interval between two successive arrivals at perihelion is 

 121 5'6 days, 1842- 1845, and the shortest 1 200*2 days, 

 1860-1871. 



In aphelion the distance of the comet from the orbit of 

 the planet Jupiter by the elements of 1875 is 0*9 15, too 

 great to allow of any violent perturbation. In about 123^° 

 heliocentric longitude, and 6° 50' north of the plane of 

 the ecliptic the comet approaches the orbit of Mercury 

 within 0038 ; to bring the bodies into closest possible 

 proximity it is necessary that the planet shall arrive at 

 perihelion 125 days before the comet, and we know that 

 a very close approach to this condition took place in 

 November 1848, whereby, on the 22nd of that month, the 

 comet was brought within 0*0378 from the planet, a 

 distance of about fifteen times that which separates the 

 moon from the earth. A close encounter with Mercury 

 appears hardly possible before the year 1904. 



If the orbit of Encke's comet was fixed within its 

 present comparatively restricted limits by planetary 

 attraction, it seems quite as likely that this may have been 

 occasioned by an extremely close approach to Mercury as 

 that Jupiter at some distant period should have been the 

 disturbing agent. 



Occult ATiONS of the Planet Saturn. — We are 

 not very fortunate in this country as regards the circum- 

 stances of the batch of eleven occultations of Saturn by 

 the moon, which take place in successive lunations, com- 

 mencing on the 22nd of March next ; the only one visible 

 in England being that on the morning of the 7th of 

 August, and this will be a daylight phenomenon, the 

 sun rising, at Greenwich, more than half an hour before 

 the immersion. Of the ten occultations of the planet 

 in 1870, three were visible here, and the occultations of 

 that year possessed greater interest from the circumstance 

 of the wider opening of the rings, than those of 1876 are 

 likely to be attended with, wherever witnessed. The 

 near approach of Saturn to the moon's limb between i 

 and 2 A.M, on July 11, as viewed at Greenwich, does not 

 appear to be converted into an occultation in any part of 

 these islands. 



While writing upon occultations, a word may be said of 

 the close approach of the planet Jupiter to the star ^ 

 Scorpii, on the morning of February 28, which is entered 

 as a possible occultation in the American Ephemeris. 

 fi Scorpii is a double star, the components being of 2 and 

 6^ magnitudes, distance about 13", or according to the 

 " Melbourne General Catalogue " of 1870, the smaller star 

 follows in R.A. o'4os., and is ii"*95 north of the brighter 

 one. The apparent position of 0^ Scorpii on Feb. 27 is in 

 R.A. ish. s8m. i4-4is., and N.P.D. 109° 28' 2*1". i:\i^ Nau- 

 tical Almanac place of Jupiter, which is from Bouvard's 

 Tables, will probably require a correction of about + o*90S. 

 in R.A., in which case the conjunction of planet and star 

 would take place a few minutes after meridian passage at 

 Greenwich on the morning of the 28th or about 5h. 40m, 

 A.M., and the north limb of Jupiter is brought close upon 

 the star, but there still seems likely to be a difference of 

 some three or four seconds in N.P.D,, by which small 

 quantity the star may escape occultation. The companion 

 is too far north to be occulted. This judgment is formed 

 by a comparison of the latest published corrections of 

 Bouvard, given by the Greenwich observations, and the 

 differences between Le Verrier and Bouvard at the end of 

 1877, 



A close approach of Jupiter to this star is recorded by 

 the Chinese as early as the year a.d, 73 ; on the 12th of 

 February the planet was very near the star, four days 

 afterwards the star was seen having been previously 

 hidden by the superior brightness of Jupiter ; and the 

 Chinese also report that the planet which had been. very 

 near to /3 Scorpii A.D. 512, January 12, occulted it on the 

 17th of April following. 



PROF. STOKES ON THE EARLY HISTORY 

 OF SPECTRUM ANALYSIS 



THE following extract from a letter, relating to the 

 early history of spectrum analysis, from our highest 

 English authority on physical optics, cannot fail to 

 interest, apart from its intrinsic importance, a wide circle 

 of readers. I have therefore obtained permission from 

 Prof. Stokes to forward it to Nature. 



C. T. L. Whitmell 



"Cambridge, Dec. 23, 1875 

 . . . . " I felt that the coincidence between the dark D 

 of the solar spectrum and the bright D of a spirit-lamp 

 with salted wick could not be a matter of chance ; and 

 knowing as I did that the latter was specially produced 

 by salts of soda, and believing as I did that even when 

 such were not ostensibly present, they were present in a 

 trace (thus alcohol burnt on a watch-glass and a candle 

 snuffed close, so that the wick does nof project into the 

 incandescent envelope, do not show bright D), I con- 

 cluded in my own mind that dark D was due to absorp- 

 tion by sodium in some shape. In what shape ? I knew 

 that such narrow absorption-bands were only observed in 

 vapours ; I knew that as a rule vapours agree in a general 

 way with their liquids or solutions as to absorption, save 

 that in lieu of the capricious absorption of the vapour, we 

 have a general absorption attacking those regions of the 

 spectrum in which the vapour-bands are chiefly found. 

 Hence as the sodium compounds, chloride, oxide, &c., are 

 transparent, I concluded that the absorbing vapour was 

 that of sodium itself. Knowing the powerful affinities of 

 sodium, I did not dream of its being present in 2. free state 

 in the flame of a spirit-lamp ; and so I supposed that the 

 emitting body in the case of a spirit-lamp with salted wick 

 was volatilised chloride of sodium, capable of vibrating in a 

 specific time, or rather two specific and nearly equal periods, 

 by virtue of its sodium constituent ; but that to produce 

 absorption the sodium must be free. I never thought of 

 the extension of Prevost's law of exchanges from radia- 

 tion as a whole to radiation of each particular refrangi- 



