566 



NATURE 



[June 30, 192 1 



not all, are built up of rays. Illustrations are given 

 both from drawings and from a photograph, the pre- 

 ference being given somewhat to the former method, 

 although reference is made to the method devised by 

 Prof. Stormer of measuring auroral heights by taking 

 photographs simultaneously from the two ends of a 

 base, the inclusion of stars determining the position 

 of the aurora in space. Reference is made to the 

 exceptional occurrence of aurora in England, whilst it 

 is said that in high latitudes aurora seems to be the 

 rule, rather than the exception, when the sky is free 

 from cloud and the absence of strong moonlight per- 

 mits. It is suggested that the spectrum of aurora at 

 different heights may add to our knowledge of the 

 comfMDsition of the atmosphere and throw light on the 

 electrical conditions of the air, whilst relations to 

 wireless phenomena are also foreshadowed. The 

 occurrence of aurora, associated with the sun-spots 

 in May, although apparently of little importance in 



England, may in more northern latitudes afford useful 

 information. 



We understand that part i of vol. iv. of "Annual 

 Tables of Constants and Numerical Data : Chemical, 

 Physical, and Technological," is now ready. The 

 work since igio has been published under the 

 patronage of the International Union of Pure and 

 Applied Chemistry. Copies are obtainable from M. 

 Ch. Marie, 9 rue de Bagneux, Paris 6*. 



Mr. W. H. Robinson, 4 Nelson Street, Newcastle- 

 upon-Tyne, has just circulated a lengthy catalogue 

 (No. 3, 192 1) of upwards of 1000 second-hand books. 

 The contents are of a varied character, but many 

 items should be of interest to readers of Nature, 

 e.g. a number of books illustrated by Thomas and 

 John Bewick, folk-lore publications, and those in the 

 large section devoted to science and technology. The 

 prices asked appear very moderate. 



Our Astronomical Column. 



OccuLTATiON OF Venus. — A daylight occultation of 

 Venus will take place on Saturday morning, July 2 

 (civil reckoning). The planet's stellar magnitude will 

 be -3-9, and it should be plainly visible to the naked 

 eye, especially with the lunar crescent as a guide. 

 The following tcble is extracted from the B.A.A. 

 Journ. for May, p. 302 : — 



The times are for the centre of Venus ; they should be 

 diminished by about o-5m., owing to the error of the 

 moon's place. Venus will be just half-illuminated. 

 The occultation (disappearance and reappearance) of 

 the illuminated limb will take place about 28s. later 

 than the centre. Accurately timed observations of the 

 different phases will be of use for correcting the places 

 of moon and planet. 



Circular No. 10 of the Cracow Observatory gives 

 full details of the circumstances for about 400 stations 

 spread over Europe. These circulars are written in 

 Prof. G. Peano's "Latino sine flexione," which is 

 easily read by anyone with an elementary knowledge 

 of Latin or the derived languages. 



The Total Amount of Starlight.— Prof. Newcomb 

 pointed out the importance of ascertaining the total 

 amount of light given to us by all the stars, including 

 those that are altogether invisible as units in the 

 largest telescopes. It is only in this manner that limits 

 can be fixed to the amount of light given by the 

 fainter and more distant stars. Prof. Newcomb him- 

 self made observations for this purpose (Asirophys. 

 Journ., vol. xiv.) ; he was followed by Mr. G. J. Burns 

 (Astrophys. Journ., vol. xvi.), Mr. L. Yntema (Gronin- 

 gen Publications, No. 22), and Dr. P. J. Van Rhijn. 

 The last-named has now made a new and more com- 

 plete research (Groningen Publications, No. 31), 

 utilising the experience previously gained, and 

 analysing the total skylight into its components. An 

 artificial star of magnitude about 5 was used, formed 

 by reflection from a bulb ; its light was compared 

 with that of standard stars, and then spread out by 



NO. 2696, VOL. 107] 



changing the focus until it became equal to the sky- 

 light. The observations were made at Mount 

 Wilson ; the nearest towns were distant 13 km. and 

 26 km., and the effect of their lights was found to 

 be inappreciable above altitude 35°. Use was made 

 of the star counts in different galactic latitudes to 

 estimate the increase of starlight as the galaxy is 

 approached. The final result is that the total starlight 

 is equal to 1440 first magnitude stars (Yntema found 

 1350), and that the skylight is made up as follows : — 

 Starlight, 17 per cent. ; zodiacal light, 43 per cent, 

 (♦^his varies at different hours of the night) ; perpetual 

 aurora, 15 per cent, (it is noted that Prof. Slipher 

 found the green auroral line on all photographs of 

 the sky spectrum) ; the preceding sources scattered 

 by the atmosphere, 25 per cent. The starlight has 

 been reduced to the zenith by the application of 

 Abbot's coefficients of atmospheric absorption. The 

 starlight per square degree in various galactic lati- 

 tudes is : — Lat. 0°, 0085 ; lat. 10°, 0065 ; lat. 20°, 

 0044; lat. 30°, 0026; lat. 40°, 0015; lat. 50^, 

 0014; lat. 60°, 0012; lat. 70°, o-oii; lat. 80°, 

 o-oio; and lat. 90°, 0010 (the unit is mag. 10). 



Periodicity of Vari.\ble Stars.- — In order to facili- 

 tate further research on the cause of th^ p>eriodicity of 

 variable stars, Dr. J. G. Hagen has collected together 

 in the May number of Scientia the salient differences- 

 between the stars of period less than three months 

 and those of greater period. The short-period stars- 

 change less than 1-5 magnitude, while those of long 

 period change three or four magnitudes. For the 

 former the minima are sharp, followed by a rapid 

 recover}', while for the latter the minima are 

 flattened and the recovery relatively slower. The 

 long periods oscillate, while the short change in 

 the same direction with time. The former 

 collect about 300 days, the latter about half a day 

 and five days. The long-period stars are generally 

 orange-red in colour and are spread evenly over the 

 sky, while the short-period stars are whitish-yellow 

 and collect in the Milky Way. 



Dr. Hagen looks forward to the appearance of the 

 results of the Mount Wilson measurements with the 

 new loo-in. telescope, and hopes that it will then be 

 possible to test whether the phenomena can all be 

 explained by the theory that the variable stars are 

 binaries. 



