268 



NATURE 



[April 29, 1920 



Perhaps the most interesting application of the 

 loo-in. telescope hitherto made is that rendered 

 possible by the utilisation of Michelson's iiiter- 

 ference method for the measurement of the spec- 

 troscopic binary star Capella. The method con- 

 sists in completely covering the loo-in. mirror 

 by a screen in which are two slits, which can 

 be placed at any desired distance apart. Light 

 coming from a point source, such as a single star 

 at a very great distance from the earth, passes 

 through the two slits and is brought to focus by 

 the large mirror. A system of interference fringes 

 may then be seen under a telescopic power of 

 about 5000 diameters, which are sharply defined 

 even on a night of poor seeing. If the star is 

 single, the fringes remain visible even when the 

 slits are separated by the full diameter of the 

 lOO-in, mirror. But if the star is a very close 

 double, the fringes will disappear (assuming the 

 members of the pair to be nearly equal in bright- 

 ness) when the slits, set by observation at the 

 proper position angle, are moved apart to a 

 distance that depends upon the angular distance 

 between the star's components. ^ 



The following measures of Capella, made by 

 Dr. Anderson, indicate the possibilities of the 

 method : 



1 Tn p actice. a somewhat different technique, giving the same result with 

 higher precision, is employed by Dr. Anderson. 



When plotted, these points fall accurately on 

 an ellipse. The method, which has been tested 

 experimentally in the laboratory, not only allows 

 binaries that cannot be resolved by other means 

 to be measured with very high precision, but also 

 permits i'wice the theoretical resolving power of 

 the Hooker telescope to be attained in practice, 

 even when the seeing is poor. 



This application of the interferometer was sug- 

 gested by Prof. Michelson many years ago and 

 used by him in the measurement of the diameter of 

 Jupiter's satellites at the Lick Observatory in 

 1891. The possibility of seeing the fringes under 

 ordinary atmospheric conditions with the full 

 aperture of the Hooker telescope was demon- 

 strated by Prof. Michelson during a visit to 

 Mount Wilson last September. The method will 

 have many applications, and should be utilised 

 by observers with instruments of moderate aper- 

 ture who wish to resolve close doubles and to 

 increase greatly the precision of their measures. ^ 



From this record of the preliminary tests of the 

 Hooker telescope it will be seen that in light- 

 collecting power, in the increased scale and im- 

 proved photographic definition, and in the added 

 possibilities of optical resolution attained through 

 the application of Michelson's method, the new 

 instrument has not disappointed our hopes. We 

 must now endeavour to utilise these advantages 

 in the extension and development of our re- 

 searches on stellar evolution and the structure 

 of the universe. 



'■J For an account of this method, see Michelson, "On the Application of 

 Interference Methods to Astronomical Measurements," Phil. iMag..]vi\y 



Artillery 

 By Sir George 



'"'PHE religious attachment of the officer of 



J- artillery to the practice of his predeces- 

 sors " was described by Benjamin Robins about 

 1740, and his attachment persisted with un- 

 impaired devotion right up to the war. There he 

 found himself outclassed at the outset, out-gunned 

 and Qut-gunnered ; the little artillery he took out 

 was small and puny, and not of the right sort re- 

 quired— "pas de celle qu'il faut." Our Artillery 

 Authority cannot be said to have understood what 

 it spelt, the word " artillery." 



On the assumption of our politicians that this 

 country was never going to war again, an 

 interdict had been laid on England of seven lean 

 year^ ; and when they were up, the lean years got 

 an extension leading right up into the war. 



A well-disciplined Army Council had been 

 formed, obsequious to the Minister, with instruc- 

 tions to resist all suggestions of military progress 

 — housed in a magnificent new palace in Whitehall, 

 the bii,rracks of an army of War Office clerks, pro- 

 vifi^. out of a reduction of ;the Regular soldiers. 



Temple of Victory it cannot be callea. The 

 stone slab over the portal is still blank, ready to 

 NO. 2635, VOL. 105] 



Science. 



Greenhill. 



F.R.S. 



receive the appropriate motto, with no derange- 

 ment in the epitaph : 



PACEM para BELLUM SI VIS. 



The mentality of the Army Council can be 

 glimpsed in its attitude to Flight in warfare. The 

 Wright brothers framed on their wall the 

 egregious answer of the Secretary in the oflficial 

 jargon : " I have nothing to add to my last letter 

 to you. The War OflSce is not disposed to enter 

 into relations with any manufacturers of air- 

 planes." 



This was in March, 1913, and only the next 

 year we were running the risk of our whole Army 

 being completely surrounded, with no airmen to 

 scout for us. The evil name " Maubeuge " would 

 have been written on our history as indelibly as 

 "Jena" and "Sedan" were on others. No 

 wonder the German squadrons could fly all over 

 England and London with impunity, in the face of 

 all our air defence. 



The belated arrival, in the war of the Tank is 

 anotner similar story. Military prejudice pre- 

 ferred to muddle along in a Stalemate" of trench 



