48 



DISCOVERY 



apart from each tribal dialect. On the other hand, a 

 foreigner like St. Boniface must have been sufficiently 

 conscious of the distinction of his own language from 

 that of his converts, and was in a better position than 

 a native to visualise the people as a separate entity. 

 And there is considerable evidence to show that the 

 learned term IhcotiscHs, which in its earliest use is 

 always applied to the language, and on the analogy 

 of which the German diutisc was formed, was an inven- 

 tion of the Anglo-Saxon mission, and formed on the 

 model of their own ]'cociisc — i.e. of, or belonging to, the 

 jx-ople. At all events the first use of the term occurs in 

 786 in a Latin document referring to an English synod. 

 From 788 onwards the Latinised term theoliscus is 

 common in Germany in the special sense of applying 

 to the conglomeration of German dialects as a whole 

 in contrast to Latin. It seems quite likely, as Braune 

 suggests, that the first incentive to its use with this 

 particular meaning went out originally from the Anglo- 

 Sa.xon Wynfrith, who as spiritual head of the German 

 Church had need of a single term to denote the 

 language of his whole people. 



Gravitation and Light 



By H. Spencer Jones, M.A., F.R.A.S. 



CMel Assistant at the Royal Observatory, Greenwich 



During the past few months, much attention has been 

 given in the Daily Press and elsewhere to what is known 

 as the " Generalised Theory of Relati\dty." The 

 average person who has endeavoured to follow the 

 discussion which has ensued has doubtless acquired 

 but a very hazy notion of the nature of this theorj' : he 

 will probably have concluded, however, that so much 

 discussion can only have been caused by a theory 

 which marks a definite epoch in scientific thought. 

 The theorj', which has been developed by a Continental 

 physicist, Professor Albert Einstein — whose name is 

 now a household word — does, in fact, revolutionise our 

 conceptions of space and time. 



The present article is not concerned with this theory, 

 but with the discovery which attracted general atten- 

 tion to it and started the discussion. This was the 

 proof that the path of a ray of light is bent when it 

 passes near matter. WTiether Einstein's theory is true 

 or not — and men of science arc divided into two 

 camps on that question — it cannot be denied that this 

 result has been definitely established by recent astro- 

 nomical observation. Its importance is due to the 

 fact that it is the first new thing which we have been 

 able to learn about gravitation since the master mind 



of Isaac Newton enunciated the law of gravitation 

 more than two hundred years ago. 



Newton's law of gravitation states that any two 

 particles of matter attract one another with a force 

 which is proportional to the product of their masses, 

 and inversely proportional to the square of their dis- 

 tance. Expressed mathematicallj', if m, m ' are the 

 masses of the two particles and r their distance apart, 

 the force of attraction between them can be written 

 kmm^//', where A is a constant called the " constant of 

 gravitation," which is the same for any two particles. 

 Newton showed that this law was satisfied by the 

 motions of the planets round the sun, the laws of 

 which were stated by Kepler, and also that it governed 

 the motion of the moon around the earth. The in- 

 creasing refinements of astronomical observation have 

 since served to increase our confidence in the law and 

 its universality, and the discovery of the planet 

 Neptune, the story of which is so well known, was a 

 further triumph for the law. 



Yet, strange to say, until a few months ago our 

 knowledge of gravitation still stood where Newton 

 left it. Although so universal, and governing every 

 action of our daily life, it has remained aloof from the 

 other forces of Nature. We have gained more or less 

 clear conceptions of what constitutes sound, heat, light, 

 electricity and magnetism, and of the properties of 

 material bodies such as elasticity, cohesion, rigidity, 

 etc., but only now has the barrier of gravitation been 

 broken down. 



It is a tribute to the sagacity of Newton that he 

 foresaw the manner in which this barrier might be 

 broken down, for in his Opticks we read : " Querj' i. — 

 Do not bodies act upon hght at a distance, and by their 

 action bend its rays, and is not this action strongest 

 at the least distance ? " Newton, of course, believed 

 light to consist of minute corpuscles emitted by a body, 

 and it was therefore to him a logical possibility' that 

 the corpuscles might be attracted bj' matter and fall 

 towards it in a similar manner to that in which the 

 bullet from a rifle falls towards the earth. However, 

 Newton was not in a position to establish his quer\'. 

 That was left for the twentieth centur\'. 



We will explain first the observed phenomenon, and 

 then the manner in which it was established. 



In Fig. I, suppose s to represent the sun, e the eye of 

 P' 



an observer on the earth, and p a starVhich is seen by 

 the observer close to the sun's edge. The figure is not 



