NA TURE 



[August 12, 192: 



on the calculation of absolute magnitudes of stars as 

 determined from the apparent magnitudes (which are 

 known) and from the trigonometrical parallaxes (also 

 known) obtained from one or other of the methods 

 previously described. The equation for this computa- 

 tion is as follows : 



Absolute Mag. = Apparent Mag. + 5 + 5 log (Parallax). 



Stars of the same type of spectrum but of different 

 absolute magnitudes are then compared with one 

 another and the relative intensities of selected pairs 

 of lines carefully measured. Curves are then drawn 

 showing as ordinates the observed differences of inten- 

 sit it-s for each selected pair of lines, and as abscissae the 

 absolute magnitudes. 



With these data it is a simple matter to determine the 

 parallax of any star. Thus, it is only necessary to (1) 

 determine first its type of spectrum, (2) measure the 

 differences of intensities of certain lines in it and refer 

 these values to the curves for that type : the next step 

 is to (3) note from the curve the corresponding absolute 

 magnitude, and lastly (4) determine the parallax from 

 this absolute magnitude by means of the same formula 

 as given above but arranged in a different order, thus : 



5 log (parallax) = Absolute Mag. - Apparent Mag. - 5, 



in which all the members on the right-hand side of the 

 equation are now known quantities. 



Thus a single photograph of the spectrum of a star is 

 sufficient for the determination of the star's distance. 

 Naturally greater accuracy is obtained when more than 

 one photograph is examined and several pairs of lines 

 in them are used, but this involves very little extra 

 labour. 



The rapidity with which the determinations of 

 parallax can be secured, when once the fundamental 

 curves are formed, is far in excess of that of the older 

 methods. The large , powerful instruments of the 

 present day are capable of photographing the spectra 

 of very faint stars, so that a rapid survey of the whole 

 heavens, at any rate to stars of about magnitude 6-5, 

 will be accomplished in the near future. 



At the recent meeting of the International Astro- 

 nomical Union in Rome, great attention was paid to 

 organising this work on an international basis. The 

 Parallax Commission pointed out that there is a large 

 amount of latent information regarding stellar distances 

 in the long series of spectrograms obtained for other 



purposes at many observatories, and it is to be hoped 

 that these data would be utilised. 



A year ago the spectroscopic determinations of 

 parallax were confined entirely to the United States at 

 the Observatories of Mount Wilson and Harvard 

 College. The Astrophysical Observatory at Victoria, 

 B.C., now proposes to examine their slit spectrograms 

 for this purpose. 



In this country the only observatory occupied at 

 present with this work is the Norman Lockyer Observa- 

 tory at Sidmouth. For more than a year the large 

 collection of spectrograms has been undergoing measure- 

 ments in this connexion, and a large number of new 

 photographs has been taken. An interesting point in 

 this observatory's work is that the measurements of 

 the intensity differences between pairs of lines are being 

 determined by a method originated by the writer, 

 which is different from either of those used at the 

 American observatories. Thus an independent check 

 on the American results is rendered possible. 



It is necessary to point out, however, that this 

 research on so large a scale could not have been under- 

 taken had it not been for the opportune assistance 

 rendered by the Department of Scientific and Industrial 

 Research. This Department appointed Mr. W. B. 

 Rimmer, D.I.C., in July 1921 as a research assistant, 

 and his appointment was due to terminate towards 

 the latter end of this year. It is with very great 

 satisfaction that it may now be stated that it has 

 been extended to September of next year. The 

 work is so far advanced that now most of the 

 fundamental curves are completed. It is hoped, 

 therefore, to publish shortly the spectroscopic paral- 

 laxes of about 500 stars, followed after a short interval 

 by another 500. 



It is satisfactory, therefore, to record that in this 

 new impetus given to the investigation of the distances 

 of the stars, this country is taking a part, and it is hoped 

 that other observatories here which have useful material 

 will join in and discuss it from this point of view. 



This line of research should also provide an interesting 

 field of work for the amateur astronomer. The instru- 

 mental equipment required need be only on a moderate 

 scale, for a five-inch telescope, fitted with a suitable 

 prism, would meet the case, if a larger one were not 

 available. It is a definite and straightforward piece 

 of research which would be a valuable contribution to 

 astronomy. 



Short-wave Directional Wireless Telegraph. 1 

 By C. S. Franklin. 



DIRECTIONAL wireless telegraphy is by no 

 means a new development, for Hertz made 

 use of reflectors at the transmitting as well as the 

 receiving ends in order to augment the effects, and 

 to prove that the electric waves which he had dis- 

 covered obeyed, to a considerable degree, the ordinary 

 optical laws of reflection. Senatore Marconi, in his 

 earliest endeavours to develop a telegraph system 

 using electric waves, also employed reflectors to in- 

 crease the range and get directional working. 



NO. 2754, VOL. I 10] 



The discovery by Marconi of the great increase of 

 range obtained by the use of longer waves, and the 

 earthed vertical aerial, practically stopped develop- 

 ment on directional lines for the time being. The 

 demand of the time was for increased ranges ; and as 

 the first practical application of wireless telegraphy, 

 namely, working to and between ships, required "all 

 round " working, there was very little call for directional 

 systems. 



To-day the range has arrived at the maximum 

 possible on the earth, and the wave-length has in- 

 creased to such an extent that the frequencies pro- 



