June 6, 1889]] 



NATURE 



139 



this observation alone it would seem extremely probable that when 

 a comet enters our system for the first time it simply means that 

 a swarm of meteorites in that part of sjiace throui^h which the 

 sun was passing at the lime began to feel the sun's attraction, 

 and ultimately became a member of our system, and also that 

 when we see the appearance which we call a nebula in space, 

 since its spectrum is the same as the spectrum of a comet, the 

 nebula is simply a swarm of meteorites if it be true that a comet 

 is a swarm of meteorites. 



These nebulous masses, visible in all parts of the heavens, 

 but in some parts of the heavens very much more numerously 

 than in others, were very early observed and imagined to be 

 very different in nature from the so-called fixed stars. 



Ptolemy was the first to point out, when he was making his 

 map of the stars that there were certain "cloudy" stars of 

 which he gave 5 on his map, and Tycho Brahe, whose work 

 was done before the invention of the telescope, although he did 

 not notice any bodies which we now class as nebulw, was firmly 

 convinced that that nebulous luminosity, which we call the 

 Milky Way, was something entirely different in its nature from 

 the stars. He imagined it to be what he called an ethereal 

 essence, a sort of fire mist, so that when in his time, in the year 

 1572, a new star appeared, he supposed it to be a considerable 

 agglomeration of this ethereal fluid. Galileo was able to show 

 that the Milky Way, the "ethereal substance" of Tycho, was 

 only an appearance due to enormous numbers of stars lying in 

 the same visual ray, the stars of which the Milky Way is 

 composed can indeed be seen with very small optical power. 

 It was not till 1612, a few years after the introduction of the 

 telescope, that we got the first real definition of a body which 

 we now call a nebula. 



The fir.-t observation we owe to Simon Marius, who stated 

 that some of the bodies visible exactly resembled the appearance 

 produced by the flame of a candle seen through horn. It was 

 not till 1656 that the nebula in Orion was discovered, although 

 now to the trained eye it is very easily visible, so that it seems 

 rather wonderful that it was not discovered before. In 1714, in 

 England, attention began to by paid to these bodies, but it was 

 not until the time of Sir Wm. Herschel that the most magnificent 

 revelations were made. He « as the first to construct very large 

 telescopes, the function of very Irrge telescopes being to collect 

 light, so that objects which appear to the eye as excessively 

 dim may be brought into full visibility. 



After not only Sir Wm. Herschel but his son, Sir John 

 Herschel, had accumulated vast stores of facts, Lord Rosse 

 took up the story, and made a telescope very much more power- 

 ful than any which had been employed by the Herschels. His 

 telescope has a light-grasping power co.npared with the eye of 

 130,000. One of the results of Lord Rosse's work to which we 

 need here refer is the idea that in a great many bodies which 

 had been classed as nebulae this enormous increase of optical 

 power suggested that we were only dealing with very distant 

 clusters of stars. 



Lord Rosse was able to get the suggestion of " resolvability " 

 in so many bodies which had been classed as nebula: by Sir 

 Wm. Herschel and others, that gradually the idea came to be 

 held that the most nebulous nebula, if we could get sufficient 

 optical power to bear upon it, would be broken up into stars, 

 just as certainly as the Milky Way had been. 



This Wduld mean that the nebulre were simply clusters of stars 

 so infinitely remote from our ken that even with the power of 

 Lord Rosse's instrument they put on the appearance of an 

 ethereal essence. 



This was the general opinion in 1864, in the early days of spec- 

 trum analysis, when IJr. Huggins turned his spectroscope one night 

 to one of the planetary nebula\ At first he thought that something 

 had gone wrong in the apparatus because he could only see a bright 

 line instead of the usual sort of spectrum obtained from a star. 

 The spectroscope, however, was doing its level best, and the 

 cause, the anomaly, was really that the nebula gave out mono- 

 chroaialic light. 



In some cases another line was seen, stated to be due to 

 hydrogen. It now appears that the dispersion employed was so 

 small that the discoverer had no right to allocate any line, so 

 that it is fortunate that other observers have since shown that 

 there is another hydrogen line visible. 



Dr. Huggins came to the conclusion that the first line was very 

 nearly, if not exactly, in the position of the chief line seen in 

 the spectrum of nitrogen, and the suggestion was therefore 

 made that these nebula; were masses of nitrogen and hydrogen 



gases mixed, or, if not nitrogen, some constituent of nitrogen 

 mixed with hydrogen. That result made the idea of Lord 

 Rosse concerning the possibility of the resolvability of nebula; 

 into stars untenable. We had to consider from that time that 

 the light of the nebula: came from a gas, and hence it was held 

 that the nebulcc were masses of gas. 



Another explanation of the origin of the green line has 

 already been given. If we study the spectrum of mag- 

 nesium we find a very bright fluting with its less refrangible 

 edge absolutely in the position of the green line with the 

 dispersion generally employed ; in nebula: and in comets the 

 same line appears, if, as 1 said before, Dr, Iluggins's observa- 

 tions are to be relied upon. 



We are therefore justified in holding the view that nebula.-, . 

 like the comets, consist of meteorites. •■■» 



J. NOKMAN LOCKYEK. ^ 



{ To be continued. ) 



THE ANNUAL VISITATION OF THE GREEN- 

 WICH OBSER VA TOR V. 



'X'HE Report of the Astronomer-Royal to the Board of Visitors 

 ■*" of the Royal Observatory, Greenwich, was received at the 

 annual visitation on Saturday last, June i. 



As regards buildings, it is noted that the new 18-foot dome is 

 completed, together with the photographic dark rooms, in pre- 

 paration for work with the 13-inch photographic equatorial 

 which is to be erected this yeai^ As regards transit-circle 

 observations, we read : — 



" The regular subjects of observation with the transit-circle 

 are the sun, moon, planets, and fundamental stars, with other 

 stars from a working catalogue, which includes all the stars in 

 Groombridge's Catalogue and in the Harvard Photometry not 

 observed since 1867, and a selection from Piazzi's Catalogue. 

 Ten close circumpolar stars taken from the Co)tnaissaiice dcs 

 Teinps, or from M. Loewy's list of stars for longitude determina- 

 tions, have been observed regularly, in addition to the four 

 standard azimuth stars. The observation of these close circum- 

 polars has been much facilitated by the adoption (since 1889 

 January i) of the method used by the officers of the French 

 Service Geographique, which consists in making a number of 

 bisections of the star with the R.A. micrometer during its 

 transit, the exact time for each bisection being recorded on the 

 chronograph. The Annual Catalogue of stars observed in 1888 

 contains about 1820 stars. 



" Special attention has been given to the observation of the 

 minor planet Iris and comparison stars in connection with the 

 determination of its parallax at the late favoural)le opposition, 

 eighteen observations of the planet and 113 of twenty-eight 

 comparison stars having been made last autumn." 



As regards computations, the transits have been completely 

 reduced so as to exhibit mean Right Ascension 1889 January i, 

 and also the circle observations to exhibit mean North Polar 

 distance for the same period. Two determinations of the astro- 

 nomical flexure of the transit-circle telescope have been made 

 since the last Report, the resulting values being o" 08 and o'''52. 

 It has been found that the correction for discordance between re- 

 flection and direct observations of stars was erroneously applied 

 in 1887, and hence the results for colatitude and for position of 

 the ecliptic are also erroneous. The correct values are now 

 given, with those recently found for 1888. 



The ecliptic investigations from 1877 to 1886 have been re- 

 vised to reduce the results to the same system of flexure, R — D 

 correction, refraction and colatitude ; so the computations for the 

 ten-year Catalogue, containing 40,000 observations of 4059 stars, 

 are now practically complete. 



It has been found that the mean error of the moon's tabular 

 place (computed from Hansen's lunar tables with Newcomb's 

 corrections) is -f 0090?. in R.A., and + i"'2i in longitude, as 

 deduced from seventy-four meridian observations in 1888. The 

 mean error in tabular N.P. D. is - l"'i9, indicating that the 

 mean of the observed N. P.D.'s is too. great. A number of 

 altazimuth observations has been made and reduced to April 8, 

 so as to exhibit errors of moon's tabular R. A. , N . P. D., longitude, 

 and E.N.P.D. 



The object-glass for the new 2S-inch refractor is now being 

 worked, and, as it is to V^e of a special form, equally suitable for 

 photographic and eye observations, a'l experimental object-glass- 



