424 



NATURE 



[July 26, 1917 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed hy his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications,] 



Visibility of Interference Fringes and the Double Slit. 



The writer has found the following simple arrange- 

 ment well adapted for the study of the visibility of 

 fringes arising from a double slit and a " source " slit 

 of variable width. A double slit, ruled without ^ny 

 special care on a piece of old photographic negative, 

 was placed on the table of a spectrometer after the 

 usual adjustments had been made. With this arrange- 

 ment and a sodium flame (Meeker burner) as the 

 source of light, no difficulty was experienced in observ- 

 ing the disappearance and reappearance of the fringes, 

 with gradually decreasing visibility, some seven times. 



In the experiment as ordinarily performed (V. 

 Mann's "Manual of Advanced Optics," p. 27) the 

 source slit is at such a distance from the double slit as 

 to render the experiment impracticable, or at least very 

 inconvenient, in many laboratories. Ordinarily, too, a 

 strong source of light is used, whereas the above 

 arrangement permits the use of a monochromatic 

 source. It provides, further, a very simple experiment 

 by means of which the student beginning the study of 

 advanced optics may obtain concrete ideas on the 

 somewhat difficult subject of visibility. With a little 

 practice, estimates of the visibility at successive stages 

 may be made, and the corresponding visibility curve 

 plotted. 



To make quantitative measurements, a graduated 

 wheel was attached to the slit of an ordinary Wilson 

 spectrometer, and afterwards calibrated by the aid of 

 a travelling microscope. By this means the width of 

 the slit corresponding to the places of disappearance 

 of the fringes or to any stage of visibility could be 

 read off directly, and in a short time a complete set 

 of measurements taken. The following readings will 

 give an idea of the quantitative value of the experi- 

 ment : — 



Width of double slit ((^) =0-903 mm. 



Focal length of lens of collimator (/) = 166 ,, 



Mean value of increase in slit width 

 {w) for successive orders of zero 

 visibility =o"io7 „ 



From which X = — =0*000582 mm. 



The quantity w is accurate to about i per cent. A 

 more accurately calibrated spectrometer slit than was 

 at the disposal of the writer would permit doubtless of 

 greater accuracy in the measurements. 



J. K. Robertson. 



Queen's University, Kingston, Canada. '- 



Relations between the Spectra of X-Rays. 



KossEL has found the following relations between 

 the frequencies of the X-ray spectra : — 



La=Kp-Ka (A) 



Ma = Lv - La (B) 



As the result of recent measurements, it is known 

 that all these series consist of many more lines. 

 According to T. Maimer the relation (A) of Kossel 

 must take the form : — 



Lai ~ ^Pl ~ ^"2 •* • • • • (^) 

 Adopting the values for the wave-length given by 

 NO. 2491, VOL. 99] 



M. Siegbahn {Jahrb. d. Radioakt. u. Elektr.), we 

 have, moreover, instead of (B), 



Mp = Lyi - L0, (2) 



My, = Ly2 - L^i (3) 



I will here also remark that the following relations 

 hold very exactly through all the elements : — 



La2_ Lpo — L^i Lyi 



1 - Lyi ) 



(4) 



where A is a constant. 



In order to account for these relations, especially 

 (4), Bohr's theoretical formula should be modified as 

 follows : — 



,._..( (N-C,y <N-C,)^ 



^^1 



N being the atomic number, tij and tXj certain in- 

 tegers. It should be supposed that N — C, and N — C^ 

 represent the numbers of electric quanta contained 

 in the "effective" nucleus charge. The curve in 

 Moseley's diagram shows further that /x, and /*2 are 

 not absolute constants, but vary gradually from 

 element to element. Jun Ishiwara. 



Physical Institute, Sendai, Japan, April, 1917. 



METEOROLOGY AND AVIATION. 



A RECENT lecture by Lord Montagu of 

 Beaulieu to the Aeronautical Society has 

 directed attention to the possibility after the war 

 of conveying- mails and passengers, and perhaps 

 goods, from place to place by aeroplanes. In 

 suitable weather such transit should present no 

 difficulty save that of expense, provided that land- 

 ing places can be found in such positions that the 

 stages may not be too long, but it is obvious that 

 the weather is, and must remain, a very important 

 factor for many years to come. 



Since the foundation of the Meteorological 

 Office under Admiral Fitzroy a large part of its 

 business has been the issuing of storm warnings 

 at certain selected coast stations for the benefit of 

 shipping ; and there is no doubt that such warn- 

 ings during the fifty years or so in which they 

 have been issued have been of great use, and 

 indeed are so still. But the gradual displacement 

 of sails by steam and the increase of size, with the 

 greater trustworthiness of the engines, have ren- 

 dered vessels far less dependent upon the weather 

 than they were in Admiral Fitzroy 's time, and in 

 these days it is seldom that any regular passenger 

 boat fails to make its passage, though it may be 

 more or less delayed by bad weather. The case 

 is likely to be different with aeroplanes if they are 

 to take the place of mail steamers, and a heavy 

 responsibility will be thrown upon the Meteoro- 

 logical Office or upon whatever body undertakes 

 to issue forecasts for their guidance. 



The kinds of weather inimical to aviation are too 

 much wind, low cloud, and fog, and of these fog 

 IS perhaps the worst, as it is also in the case of 

 shipping. The ways in which wind affects an 

 aeroplane are various. There is the difficulty of 

 starting and landing, but the days on which this 

 is serious are not numerous, even in a windy 

 country like England. But still there are days 

 when landing is unsafe, and it is the misfortuae 



