December 27, 1894] 



NATURE 



201 



Several points of great interest are touched upon by 

 Prof. Holden, among which is a brief discussion of the 

 dimensions of the smallest object on the moon which can 

 be registered on the photographic plate by the 3-foot 

 refractor. From this we learn that a crater on the 

 moon which is less than one-tenth of a mile in diameter 

 will form an image which is about the same size as the 

 grains of silver in the photographic film, and cannot in 

 general be distinguished. Craters not more than o'3 and 

 o'i5 English miles in diameter, however, have been de- 

 tected already. Prof. Holden concludes that for further 

 advances in lunar photography it will be necessary 

 to employ plates of greater sensitiveness so as to shorten 

 exposure, and also plates in which the grain is finer. 

 Workers in all departments of celestial photography have 

 felt the need of such improvements, and, as Prof. 

 Holden remarks, " future improvements depend more 

 upon the manufacturer of plates than upon the as- 

 tronomer who uses them." 



Prof Weinek's concise descriptions of the lunar 

 formations figured in the volume, and his account of 

 the new features so far discovered, leave nothing to be 

 desired. Observers of the lunar surface may take con- 

 solation in the fact that even yet they are not in danger 

 of being entirely superseded by photographic methods, 

 for, as Prof Weinek points out, " both methods must be 

 perfected, and each must support the other." It is worth 

 remark here, however, that enlargements recently made 

 of lunar photographs taken at the Paris Observatory seem 

 to mark a clear step towards perfection. (See page 207.) 



I'rof. Keeler's work on the spectra of nebula: during 

 his connection with the Lick Observatory, may fairly be 

 said to mark the commencement of a new era in the 

 history of the spectroscope as an instrument of precision. 

 The observations were undertaken in the first instance 

 at the suggestion of Dr. Huggins, who appealed to the 

 Lick astronomers in 1890 in connection with the discus- 

 sion as to the origin of the chief nebular line. It 

 uas found possible to use the third and fourth order 

 spectra of a grating spectroscope with advantage, and 

 even then the spectra were " by no means extremely 

 feeble." Former work left the wave-lengths of the 

 nebular lines uncertain to at least two tenth-metres, but 

 the uncertainties now amount to only a small fraction of 

 a tenth-metre. Further, it is claimed that the observa- 

 tions of the nebulae have shown the existence of errors in 

 .Xngstnim's scale and in the wave-lengths of the reference 

 lines, so that the observations did not become consistent 

 until more reliable reference wave-lengths were deter- 

 mined by Prof Rowland. As an example of the accuracy 

 ittainable, the velocity of Venus in the line of sight was 

 tciund to be 64 miles per second at a time when the com- 

 puted velocity was 7'69 miles. 



It is not a part of our present purpose to discuss the 

 origin of the chief line in the spectrum of the ncbute, 

 but we may say that Prof. Keeler does not favour the 

 suggestion that it is due to magnesium ; but, on the other 

 hand, his measures definitely decide against the nitrogen 

 origin of the line. 



After all corrections have been applied, the normal 

 positions of the first and second lines in the nebular spec- 

 trum are stated to be 5007-05 ± 0'03 and 495902 ± 004 

 respectively, and neither of the lines is represented 

 among the Fraunhofer lines which appear in Rowland's 

 photographic map. Indeed, we are not .-;ware that 

 either of these lines has ever been recorded as an 

 absorption line in the spectrum of any celestial body 

 whatever. 



The observations have not been entirely limited to the 

 determination of the position of the chief line. It has 

 been found, for instance, that " the nebul.e are moving 

 in space with velocities of the same order as those of the 

 stars. Of the nebukr observed, that having the greatest 



NO. 13 13, VOL. 51] 



motion of approach, 40^2 miles per second, is G.C. 

 4373 ; that having the greatest motion of recession, 30'i 

 miles per second, is N.G.C. 6790. Most of the nebulse 

 have considerably smaller velocities than these." 



It might well be imagined by anyone who has 

 seen a photograph of the Orion nebula that the dif- 

 ferent parts would have a relative movement with regard 

 to each other. .Such, however, does not appear to be the 

 case, according to Mr. Keeler ; or, at least, there is no 

 relative movement greater than four or five miles per 

 second. Attempts to measure the velocity of rotation of 

 the large planetary nebula G.C. 2102 showed that there 

 was no radial motion greater than eight miles per 

 second. 



A study of the spectra of the nuclei of the planetary 

 nebuhe has led Prof. Keeler, as it has independently led 

 Prof. Pickering, to the conclusion that they are very 

 closely connected with the bright-line stars, and thus the 

 latest and most precise work goes to confirm one of 

 the fundamental points of Mr. Lockyer's meteoritic 

 hypothesis. 



With reference to the discordant accounts of the 

 spectrum of G.C. 826, to which attention was drawn by 

 myself in 1S89 (Nature, vol. xli. p. 163), it is stated that 

 Dr. Huggins's observation of a continuous spectrum in 

 1864 "was evidently a mistake," the spectrum being of 

 the usual bright-line type. 



Apparently in order to reconcile the presence of a con- 

 tinuous spectrum in such a nebula as that of Orion with 

 the idea that masses of rarefied gas were alone in question, 

 it has been suggested that this continuous spectrum may 

 really be a large number of adjacent bright lines. The 

 enormous dispersion employed by Prof. Keeler, however, 

 fails to resolve it into lines, and thus Prof. Tait's sug- 

 gestion as to the meteoritic constitution of nebute still 

 stands as the best explanation of the spectrum. 



Many other points of interest are raised by Prof. 

 Keeler's admirable work, but sufficient has been said to 

 indicate the progress which has been made in this branch 

 of celestial physics and chemistry. Although Prof. 

 Keeler has now removed to the .Allegheny Observatory, 

 his successor at the Lick Observatory — Prof Campbell 

 — has already shown himself to be fully capable of main- 

 taining the spectroscopic department of the Observatory 

 at the same high standard of efificiency. 



.\. Fowler. 



STUDIES OF A GROWING ATOLL. 



'TPHE researches of the surveying ships of the British 

 -l Navy have from time to time rendered services to 

 science no less important than those which it is their 

 function to perform for navigation. It has become an 

 established practice to encourage the surgeons of these 

 vessels to undertake scientific investigations in the 



I leisure which their professional duties frequently 

 afford, and facilities are sometimes given for a com- 



I petent man to continue such work by allowing his trans- 



. ference to another vessel when his own has to leave the 

 place where he has been working. For this the Admiralty 



( deserves credit and the thanks of those who desire to 

 see her Majesty's ships maintaining the position they 



' took up in the days of Cook, and continued through 

 the voyage of the Beagle, and the long line of ex- 



I peditions which followed it, to the voyage of the 



! Challenger. While it may not be too much to hope for 

 a renewal of special marine research by the Royal Navy 

 before private enterprise reaps the waiting scientific 

 harvest of the unknown Antarctic, we feel that too much 

 prominence cannot be given to the good work done inci- 

 dentally in the course of routine surveys. 



The hydrographer, Captain Wharton, in his preface to 

 the reports of Mr. Bassett-Smith on the .Macclesfield 



