88 I'KKSIDE.NTIAL ADDUKSS SKCTIOX A. 



our views about a propagation by spherical shells. Another one 

 is the assumption, contrary to the conclusion of Laplace, of a 

 non-instuntaneous propagation of gravitation. Another one is 

 to assume that gravitation is refracted through gravitie masses, 

 as light through refracting media. Yet another one is an altera- 

 tion in the principles of mechanics as proposed by the general 

 theoi-y of relativity, of which some of you may have heard a 

 good explanation from Professor Dalton. 



But these explanations have not the generality of Newton's 

 law : the alteration of the exponent also strongly alters the 

 results already confirmed by observation for other bodies; the 

 refraction is only to be applied to the motion of the Moon. The 

 relativity theory accounts sufficiently well for the irregularity of 

 Mercury, is approximate as to a small irregularity of Mars, but 

 does not explain the irregularity of the Moon. Notwithstanding, 

 it is this last theory which has the greatest likelihood of super- 

 seding the law of Newton. 



In short, astronomical observations created the law of 

 Newton, and, with it, gave the first and strongest impulse to 

 scientific investigation. Later on, the same observations, but 

 improved, caused a doubt to arise as to the accuracy of the law 

 and gave origin to new investigations. Man is nowadays weigh- 

 ing his knowledge, trying to separate what is hypothesis from 

 what is real, in order to reinforce the foundations of Science. 



But not only in the development of general Science has 

 astronomy assisted civilisation. Some special branches of science 

 have been strongly developed by astronomical observations. 



Excepting few data about the Sun which are given us by 

 its calorific, magnetic and electric radiations, all we know about 

 celestial bodies, such as movement, masses, physical and chemical 

 eonstitutions, is given us by the light they emit. So, optics owes 

 a great part of its development to astronomy. 



It was by his observation of the eclipses of the satellites of 

 Jupiter that Boemer discovered in 1676 that hght travels with a 

 finite velocity. However strange this statement may appear, it 

 will be recognised that this discovery was the first step towards 

 the invention of wireless telegraphy. By that time, Sir Isaac 

 Newton was working up the corpuscular or emission theory of 

 light. The discovery of Koemer — light moving as a material 

 body — forced him to modify his former views in order that the 

 principles of mechanics could be applied to light. Huygens, 

 however did not agree with Newton's views and, in 1678, 

 presented his wave theory of light. Both theories gave a bad 

 account of some luminous phenomena; so that it was easy to 

 contradict them. 



The wave theory was supported by the similarity of sonorous 

 and luminous phenomena. Curiously enough, Newton, wdio was 

 the creator of the wave theory of sound, did not wish to apply 

 the same ])rinciplos to light. So tenaciously did he defend his 



