374 



NATURE 



[December ii, 1919 



or other constructions in tidal waters built during the 

 years 1918, 1919, or even 1917. As the cxarnination 

 for sex can be conducted with speed only during the 

 breeding season, which may shortly end in some 

 localities, it is hoped that anyone knowing of suitable 

 constructions will inform the writer at the address 

 ,t,<iven below. 



Evidence of sex-change is apparontK already avail- 

 able from Gfrnmill's observations,' made so long ago 

 as 1896. Gemmill found that 3 specimens out of 250 

 examined by him were hermaphrodite. In the samples 

 quoted above no definite hermaphrodite forms were 

 found, but several were su.spected and preserved for 

 microscopic examination. These forms were, how- 

 ever, mainly male or female, and are recorded above 

 tentatively as males and females respectively. Sex- 

 rhange may be seasonal, .'is is indeed indicated by 

 Russell's observations on the sex of the common 

 limpet.* 



The sex-phenomena in the common limpet clo.sely 

 resemble in most respects those found in the slipper- 

 limpet," where the small females and large males are 

 accounted for, but in which all the tiny ones — some 

 thousands of which have now been examined — have 

 a penis. It is,, however, not impossible, on the evi- 

 dence available at the moment, that sexual dimorphism 

 without sex-change may explain the phenomena in the 

 common limpet, but this explanation does not seem 

 probable* The observations on the sex of the common 

 limpet cannot all be described here ; they will be 

 continued and completed and the results publi-ihed in 

 the Journal of the Marine Biological .'\ssociation, Pl\ - 

 mouth. Fig. i shows a length-sex analysis of the 

 sample of small limpets under i in. in length col- 

 lected at Plymouth. J. H. Okton. 



The Marine Biological Laboratory 

 The Hoe, Plymouth. 



A Tribute from Prague. 



Permit me to congratulate you upon the jubilee 

 issue of November 6, which has just reached me and 

 has been received with great pleasure, for tlie last 

 number of Nature which reached me before this was 

 that of July 30, 1914 ! 



It may interest readers of Nature to learn that 

 from that date the Austrian Government prohibited 

 for more than four years the circulation of anything 

 printed in England as a punishment for the regard 

 which, especially during the war, we have always had 

 for your country, to which, with the other .Mlies, we 

 owe our liberty. To the bodily sufferings of the war 

 was added isolation from nearly the whole civilised 

 world. 



In a year's time 1 shall celebrate the fortieth ;mni- 

 versary of my introduction to Nature, for while a 

 student of Owens College, Manchester, I purchased 

 in October, 1880, my first copy of the journal, and 

 since that time I have been an ardent reader, con- 

 tributor, and even Bohemian correspondent. The 

 reading of Nature's all-round scientific contents 

 has been one of the greatest pleasures of my 

 life in my leisure hours, and the richness of 

 information which I have gathered from k 

 nnnot be exnressed in better words than tho>e of 

 Dr. Deslandres in the jubilee number. I do not 

 wonder that all attempts at founding a similar 

 scientific and yet popular (in its best sense) journal in 

 other European countries have invariably failed, for 

 there a man of science is usually identicaj with a pro- 

 fessor (a professional worker); and though I am one 



3 "On Some Cis-s of Hermnphroditis-n »n tti*r Limp-t (Patel!a\ \,'lth 

 Observations reeardine the Inii -ence of Nutrition on S- x in the I.ii'nper." 

 Oy ri. F. Gemmill. Aunt. Anzeiger, xii., pp. 302-01, i^o^- 



'< "On th- She'I-u-nwth of tile Limpet (fatella vnlgata)." I'.y E. S. 

 Russell. Proc. Z»ol. Soc, 1909(1), p. 21^. 



NO. 2615, VOL. 104] 



myself, yet I am of the opinion that the scientific 

 character of Nature is in no small measure due to the 

 high type of British scientific amateur or student of 

 science for its own sake, which I do not find equalled 

 in any other country in the world. 



Even we scientific workers in this remote part of 

 Europe owe sincere thanks to Sir Norman Lockyer, 

 who is a brilliant representative of the " non-profes- 

 sional " English man of science, for providing us with 

 Nature and conducting it so admirably for so manv 

 years. Bohuslav Brauner. 



Chemical Laboratory, Bohemian University, 

 Prague, November 17. 



EIXSTEIN'S RELATIVITY THEORY 

 OF GRAVITATION.^ 

 II. — The Nature of the Theory. 



T N the first article an attempt was made to show 

 -*- the roads which led to Einstein's adventure 

 of thought. On the physical side briefly it was 

 this. Newton associated gravitation definitely 

 with mass. Electromagnetic theory showed that 

 the mass of a body is not a definite and invariable 

 quantity inherent in matter alone. The energy' 

 of light and heat certainly has inertia. Is it, 

 then, also susceptible to gravitation, and, if so, 

 exactly in what manner? The very precise experi- 

 ments of Eotvos rather indicated that the mass 

 of a body, as indicated by its inertia, is the same 

 as that which is affected by gravitation. 



Also, how must the expression of Newton's law 

 of gravitation be modified to meet the new view 

 of mass? How, also, must the electromagnetic 

 theory and the related pre-war relativity be 

 adapted to allow of the effect of gravitation ? With 

 the relaxation of the stipulation that the velocity of 

 light shall be constant, will the principle of rela- 

 ti\ ity become more general Snd acceptable to the 

 philosophic doctrine of relativity, or will it, on the 

 other hand, become completely impossible? 



One point arises immediately. The out-and-out 

 relativist will not admit an absolute measure of 

 acceleration any more than of velocity. The effect, 

 however, of an accelerated motion is to produce an 

 apparent change in gravitation ; the measure of 

 gravitation at any place must therefore be a rela- 

 tive quantity depending upon the choice which 

 the observer makes as to the way in which h( " 

 will measure velocities and accelerations. This i 

 one of Einstein's fundamental points. It has beei 

 customary in expositions of mechanics to distin 

 guish between so-called " centrifugal force " and 

 "gravitational force." The former is said to be 

 fictitious, being simply a manifestation of the 

 desire of a body to travel uniformly in a straight 

 line. On the other hand, gravitation has been 

 called a real force because associated with a cause 

 external to the body on which it acts. 



Einstein asks us to consider the result of sup- 

 posing that the distinction is not essential. This 

 was his so-called "principle of equivalence." It 

 led at once to the idea of a ray of light being 

 deviated as it passes through a field of gravita- 

 tional force. An observer near the surface of the 



1 The firrt nrticle appealed in Natuke of December 4 



