668 



NA TURE 



[November 18, 1922 



Perhaps the phenomenon was not seen in England 

 or even in Europe. The night may have been foggy, 

 or the transit of the meteors may have taken place 

 in the daytime there. 



It does not seem impossible that Shakespeare may 

 have heard of the display from sailors and other 

 travellers in the east when he wrote about the close 

 of the sixteenth century of certain stars shooting 

 madly from their spheres. H. Beveridge. 



53 Campden House Road, W.8, 

 October 26. 



Skin Effect in Solenoids. 



Skin effect in long, single layer solenoids wound 

 with solid round wire and used at very high frequencies 

 has been treated by Sommerfeld, 1 Lenz, 2 and Abra- 

 ham, L. Bloch, anil E. Bloch. 3 (The frequency is 

 supposed so high that the Rayleigh approximation 

 applies.) 



The last of these disagrees with the first, giving 

 the ratio of the resistance of a closely wound solenoid 

 to the resistance of the wire of the solenoid when 

 stretched out straight and used at the same fre- 

 quency as equal to 3-73, while Abraham, L. Bloch, 

 and E. Bloch obtained 3-46. The writer calculated 

 the same ratio by a different method and obtained 

 23-4 + 0-02. Going through the calculation of Som- 

 merfeld and correcting for an error in the graphical 

 evaluation of the area under Sommerfeld's curve, 

 the same result (3-41) is obtained by Sommerfeld's 

 method. For loosely wound solenoids the calcula- 

 tions of Abraham, L. Bloch, and E. Bloch, Lenz, and 

 the writer are in fair agreement. 



On reading this letter Prof. Sommerfeld has in- 

 formed the writer that he agrees with this conclusion. 



G. Breit. 



National Research Fellowship, 



Cruft Laboratory, 



Harvard University, U.S.A. 



Colour Vision and Syntony. 



After reading the letter of Dr. Edridge Green 

 (Nature, October 14, p. 513) it occurred to me that 

 the following method, involving no head movement, 

 of observing the movement of positive retinal " after 

 images " might be of interest. If, in a dark room, 

 tin eyes being in a state of " dark adaptation " and 

 one covered, a dry petrol lighter of the spring release 

 type be flashed, a fan-shaped pattern of brilliant 

 streamers will be seen. This pattern is followed by 

 a similar " after image." The " after image " im- 

 mediately begins to contract. This contraction con- 

 tinues till the after image appears as a rather thick 

 irregular line of smaller area and greater brilliancy 

 than the original pattern. The rapidity of the change, 

 and the final form varies for different parts of the 

 retina. Two points are of interest, the contraction 

 "i the image, and the increase of brilliancy. 



H. S. Ryland. 



London, S.E., 

 Oct. 16. 



Mosaic Disease in Plants. 



There has been considerable speculation recently 

 upon the cause of the so-called ' virus diseases," 

 which occur in both animals and plants, such as 

 typhus and Rocky Mountain fever in man, and 



1 A. Sommerfeld, " CJber den Wechselstromwiderstand von Spulen," Ann 

 d. Phys., 329, p. 609, 1907. 



- W. Lenz. " Uber die Kapazitat der Spulen und deren Widerstand und 

 1 bei Wechselstrom," Inn i Phys., 342, p. 923, 1912. 



I Bloi b el I Bloi li, " Radi i ti I raphie militairi ," [919, 

 E.C.M.R. 1 



" mosaic " disease in plants. These diseases are 

 supposed to be due to the presence of some ultra- 

 microscopic filter-passing organism. Many small 

 bodies, some of a granular nature, have been described 

 in connexion with these disorders, such as Rickettsia, 

 Negri bodies, etc. As regards the mosaic disease of 

 plants, L. O. Kunkel, a worker in Hawaii, last year 

 demonstrated the presence of a peculiar body of 

 amoeboid appearance in the diseased cells of maize 

 affected with mosaic. 



The purpose of this communication is to place on 

 record the discovery of apparently similar bodies in 

 the tissue of potato plants affected with mosaic, a 

 disease which, so far as the potato is concerned, has 

 become of considerable economic importance. No 

 attempt is made at present to define the nature of 

 this body, but it is hoped that further work may 

 throw more light on the subject. All that can be 

 said is that there is invariably present in the cells of 

 mosaic potato tissue, in close association with the 

 nucleus, an abnormal body which is definitely con- 

 nected with the disease. Preparations showing these 

 bodies were demonstrated at a recent meeting of the 

 Association of Economic Biologists in London. . " 



Kenneth M. Smith 



The Victoria University of Manchester. 



Einstein's Paradox. 



In his review of Bergson's new book (Nature, 

 October 14) Prof. Wildon Carr refers to " Einstein's 

 paradox," which he quotes in inverted commas as 

 follows : — " Suppose a traveller to be enclosed in a 

 cannon-ball and projected from the earth with a 

 velocity amounting to a twenty-thousandth of the 

 velocity of light ; suppose him to meet a star and 

 be returned to earth ; he will find when he leaves 

 the cannon-ball that if he has been absent two years, 

 the world in his absence has aged two hundred years." 

 It so happens that a paradox of this identical kind 

 was proposed to Einstein himself by M. Painleve at 

 the Paris conferences in April of this year. Unless 

 I have greatly misunderstood Einstein's reply, as 

 recorded by M. Nordmann in the Revue des Deux 

 Mondes of May 1, this particular paradox, arising 

 from the imaginary departure and return of an 

 observer travelling with great speed from a given 

 point and back again, was shown to be one not 

 legitimately derivable from the restricted theory — 

 the theoretical construction is not one to which the 

 transformation formulae can properly be applied 

 (pp. 146-152). 



The humble' student of relativity is therefore in 

 the position of a schoolboy who finds that what he 

 learns from one master to-day is contradicted by 

 another to-morrow. Bergson and Nordmann both 

 speak with Einstein's voice ; but whereas the former 

 apparently puts the paradox before us categorically 

 as an inescapable Einsteinian fact, the other represents 

 it as a non-Einsteinian fiction. Which of these two 

 views are we to accept ? They cannot both be true. 

 Einstein, as quoted by M. Nordmann, advanced good 

 reasons for putting the paradox out of court ; but as 

 Bergson was present at some at least of the conferences 

 it appears that these reasons did not seem to him to 

 be convincing. 



There is a certain indefiniteness of phrase about the 

 paradox as quoted above which gives rise to doubt. 

 The only observer mentioned is the traveller in the 

 cannon-ball ; and it is quite overlooked that he 

 would naturally expect the difference between his 

 time and earth time to be in exactly the opposite 

 direction — he would expect earth time to have 

 advanced by only one-hundredth of his own time. 



NO. 2768, VOL. I io] 



