July i, 1922] 



NA TURE 



so in diameter, used by Galileo in 1610, brought 

 within the range of vision stars down to magnitude 10-5, 

 numbering about five hundred thousand. The 60-inch 

 reflector of the Mount Wilson Observatory, Pasadena, 

 of which Prof. Hale is director, reveals stars of the 18th 

 magnitude, and the 100-inch carries the sounding-line 

 still further, while with both instruments many stars 

 can be photographed which the eye cannot see directly, 

 the photographic limit with four or five hours' exposure 

 being about the 20th magnitude. 



The expansion of the stellar universe as regards the 

 number of stars in it which can be seen or photo- 

 graphed represents, however, only a small part of 

 modern astronomical discovery. The test of a tele- 

 scope is its resolving power, and this is expressed with 

 sufficient accuracy by the relation $"/d, where the 

 numerator is the normal angular limit of separation 

 of a double star and d is the diameter of the object- 

 glass in inches. The separating power of the 36-inch 

 refractor of the Lick Observatory is thus o"-i4, and 

 that of the 100-inch of the Mount Wilson Observatory 

 o"-o5. By the use of the interferometer, the latter 

 limit is reduced to o"-o2, and this increase in resolving 

 power was established by observations of Capella. 

 More than twenty years ago, this star was found by 

 Campbell and Newall to be a spectroscopic binary 

 (that is, to consist of two stars in motion about a 

 common centre of gravity and so close together that 

 the system is known to be duplex only by detecting 

 differences, due to orbital movement, exhibited by the 

 composite spectrum), and determination of the orbit 

 showed that the separation of the components could not 

 exceed o"-o6. It was, therefore, within the theoretical 

 limit of separation into its components by the use of 

 Michelson's interferometer on the 100-inch telescope of 

 the Mount Wilson Observatory. Observations made at 

 the end of the year 1919 and the beginning of 1920 

 confirmed the accuracy of this conclusion, and the 

 distance between the two stars of the pair was found 

 on several occasions to be about o"'045. 



The capacity of the interference method was thus 

 established by these observations with a test-object 

 among the stars. Thirty years previously the method 

 had been used to determine the diameters of Jupiter's 

 chief satellites, but it was only when the 100-inch 

 telescope had been completed that Prof. Hale suggested 

 the application of the principle to the measurement of 

 diameters of stars — a more difficult problem than 

 that of separating close doubles. Prof. Eddington, in 

 his presidential address to Section A of the British 

 Association in 1920, gave the probable angular dia- 

 meters of some stars and remarked that " the star 

 with the greatest apparent diameter is almost certainly 

 Betelgeuse, diameter o"-o5i." Measurements with 

 NO. 2748, VOL. I io] 



the 20-foot interferometer on December 13, 1920, 

 gave an apparent diameter of o"-o47, which is as 

 striking a confirmation of theoretical deduction by 

 observed result as that represented by the discovery 

 of the planet Neptune. The parallax of Betelgeuse is 

 uncertain, but there are reasons for believing it to be 

 about o"-o2, which would make the diameter of the 

 star about 215,000,000 miles, or 250 times greater than 

 the diameter of the sun. Antares has similarly been 

 found to have a diameter of 400,000,000 miles and 

 Arcturus of 21,000,000. These stars are in an early 

 stage of stellar evolution — attenuated masses of matter 

 low down on the ascending side of the temperature 

 curve of Lockyer's meteoritic hypothesis — and from 

 " giants " they will be transformed to " dwarfs " as 

 they contract and increase in temperature. 



Prof. Hale devotes particular attention to the various 

 stages of growth and decay as indicated by modern 

 studies of stellar types, and shows that they afford no 

 direct evidence in favour of Laplace's theory of the 

 formation of planets in our solar system. Recent in- 

 vestigations have truly revealed " The New Heavens," 

 which he describes so clearly and illustrates so at- 

 tractively with some of the most remarkable astronomi- 

 cal photographs ever obtained. In his last chapter, 

 entitled " Cosmic Crucibles," he deals particularly with 

 the sun as a star and some of the discoveries in the 

 field opened by him by means of the spectroheliograph, 

 which enabled him to prove that every large sun-spot 

 is an electric vortex producing a magnetic field. As 

 helium was discovered by Lockyer in the sun long 

 before it was isolated on the earth, so in the labora- 

 tories of the heavens conditions are now continually 

 being studied which not only enlarge our conceptions 

 of the universe but also provide physicists and chemists 

 with results of outstanding interest and value. No 

 one is more competent than Prof. Hale to survey this 

 great territory of which he is the leading pioneer 

 explorer, and his account of the methods used to 

 examine it and the rich store of new knowledge 

 gathered from it, makes as fascinating a scientific 

 story as ever was told. R. A. G. 



Index Animalium. 

 Index Animalium. A Carolo Davies Sherborn. 

 Sectio Secunda 1801-1850. Part 1 : Introduction, 

 Bibliography and Index A-Aff. Pp. cxxxi + 128. 

 (London : British Museum (Natural History), 1922.) 

 205. 



WE congratulate Mr.C. D. Sherborn on the appear- 

 ance of the first instalment of the second part 

 of his great work. Since the first part was published in 

 1902, zoologists have eagerly awaited its continuation, 



