474 



NATURE 



[February 14, 1918 



Lkkds Astronomical Society. — The Journal and 

 Transactions of this society for the year 19 16 has been 

 received. The number of members was fifty-two, and 

 in view of the prevailing conditions, an average at- 

 tendance of fouiteen may be taken as an indication 

 that the meetings continued to be interesting and help- 

 ful. Numerous observations of interest are recorded, 

 and among the contributed papers, one by the Rev. 

 I. Carr-Gregg on "The Invisible Universe," and 

 another on "Sir William Herschel," by Miss C. A. 

 Barbour, call for special mention. The editor is Mr. 

 C. T. Whitmell, who has also made numerous con- 

 tributions. 



WAVE-LENGTHS OF HELIUM LINES. 



ON account of its great intensity and the convenient 

 distribution of the lines, the spectrum of helium 

 furnishes a valuable source of standard wave-lengths 

 in spectroscopic and optical work. A new series of 

 determinations of the wave-lengths of the brighter lines 

 which has been made by Mr. P. W. Merrill at the 

 U.S. Bureau of Standards, Washington {Astrophysical 

 Journal, vol. xlvi., p. 357, December, 1917), will there- 

 fore be generally welcomed. The highest possible pre- 

 cision has been aimed at, and as lines belonging to 

 all the six series which constitute the spectrum of 

 helium were included in the measurements, the new 

 wave-lengths will also provide valuable data for com- 

 putations of theoretical interest. 



An interferometer of the Fabry and Perot type was 

 used, and nine of the lines were compared directly with 

 the fundamental standard — the red line of cadmium — 

 by photographing the helium and cadmium spectra 

 simultaneously on the same plate. Other wave-lengths 

 were then determined from photographs of the helium 

 spectrum alone. The adopted values for the twenty- 

 one lines measured are given in the appended table, 

 which also includes the values given bv previous 

 observers. The values given by Lord Rayleigh (two 

 sets) and Eversheim were derived from interferometer 

 observations, but those by Runge and Paschen were 

 determined in the more usual way from grating photo- 

 graphs ; the latter have been corrected from Rowland's 

 scale to the international scale in order to make them 

 •directly comparable with the other values. 



Wave-lengths of Helium Lines {in LA.). 



In the case of double lines the wave-lengths are those 

 of the stronger components. From the general agree- 



NO. 2520, VOL. 100] 



nient of individual determinations it is considered prob- 

 able that the error is .in no case so much as 0-003 '"^-j 

 and that in most cases the errors are smaller than that 

 amount. It is shown that the Kayser and Rung^ 

 formula for spectral series, based upon three consecutive 

 lines, will not reproduce accurately even the next mem- 

 ber in anv one of the six helium series. 



THE CORAL-REEF PROBLEM. 

 T7ROM time to time recent work on the topograph' 

 ^ of coral-reefs has been referred to in Natlhk. 

 and the existence of submarine platforms from which 

 atolls and encircling reefs rise has been very generalh 

 demonstrated. Prof. R. A. Daly regards these plat- 

 forms as wave-cut plains, produced from coral banks 

 and volcanic isles when the level of oceanic waters 

 was lowered by ice-accumulation in Glacial times. 

 The melting of the ice caused a general submergence 

 of the platforms and of the adjacent coasts, giving rise 

 to drowned valleys and all the features that have been 

 attributed to a subsidence of the ocean-floor. The 

 existing coral-reefs are thus for him post-Glacial, and 

 grew up on the submerged platforms when warmer 

 conditions were renewed. 



In a summary of his views in Scientia (vol. xxii., 

 p. 188, 1917) Daly points out that flat, reefless banks 

 occur "in every ocean, inside and outside the tropical 

 belt . . . covered with 45 to 100 metres of water." 

 He urges that the inner walls of reefs are not well 

 graded to the floors of the lagoons, and that the upper 

 wall thus indicates a rise of water-level (whether we 

 attribute it to flooding or subsidence) since the forma- 

 tion of the level inner floor. He believes that this floor 

 is part of the platform, and is not due to infilling, 

 though it is not clear why he should demand "millions 

 of years" for such deposition within the wall (compare 

 also his paper on "A New Test of the Subsidence 

 Theory of Coral Reefs," Proc. Nat. Acad. Sci., 

 vol. ii., p. 664, 1916). He holds that "the 

 mean depths of water above the flat floors 

 of wide lagoons are nearly equal to the mean 

 depths found on reefless banks," and that there is a 

 close similarity of depth in the greater lagoons 

 throughout the reef areas of the Pacific and Indian 

 Oceans. Daly regards the reefs as "peripheral 

 growths on wave-cut platforms," those nearer the 

 centres of the platforms having been extinguished by 

 mud and sand swept over the shoals. 



On the other hand, Prof. W. M. Davis, in a series 

 of critical papers, based on a recent visit to the Pacific 

 isles, has greatly strengthened the Darwinian view. 

 Thanks largely to his reasoning, even those who 

 cannot find evidence for a general subsidence of ocean- 

 floors are inclined to invoke block-faulting to explain 

 the drowning of certain areas. Davis ("A Shaler 

 Memorial Study of Coral Reefs," Amer. Journ. Sci., 

 vol. xl., p. 223, 1915) urg-es that if the lagoon floor is 

 part of a wave-eroded plain from which the reefs rise, 

 the sea would have cut cliffs in the surviving volcanic 

 isles, the tops of which should appear as truncations 

 of the spurs that bound the subsequently drowned 

 vallevs. Such cliffs occur in Tahiti ("Clift Islands in 

 the Coral Seas," Proc. Nat. Acad. Sci., vol. ii., p. 284, 

 1916), but are very exceptional features. Davis re- 

 gards them as emphasising the general absence of 

 cliffs, even if they "are the work of abrasion during 

 the lowered sea-stands of the Glacial period " 

 (" Problems Associated with the Study of Coral-Reefs," 

 Sci. Monthly, vol. ii., p. 564). 



Davis, in his three papers in the Scientific Monthly 

 (19 11;) and elsewhere, lays stress on the mature form^ 

 of the valleys in the reef-encircled isles as indications 

 of their antiquity. These valleys cannot have been 



