Address bv Sir David Gill. 25 



form theories whether we wish it or no. These theories may 

 be useful or they may be otherwise — we have examples of both, 

 results. If the theories merely stimulate the examination of 

 facts, and are modified as and when the facts suggest modifica- 

 tion, thev may be erroneous, but they will still be beneficial ; 

 they may die, but they will not have lived in vain. If, on the- 

 other hand, our theory be supposed to have a truth of a superior 

 kind to the facts, to be certain independently of its exemplifica- 

 tion in particular cases ; if, when exceptions to our propositions- 

 occur, instead of modifying our theory^ we explain away the facts,, 

 our theory then becomes our tyrant, and all who work under its 

 bidding do the work of slaves, they themselves deriving no' 

 benefit from the results of their labours." 

 This seems to me one of the most thoughtful and best expressed 

 summaries of the proper scientific use of the imagination with which-. 

 I am acquainted, and it indicates the slow and gradual process by 

 which alone the laws of Nature can be traced. 



Take, for example, the history of Newton's discovery of the- 

 law of gravitation. 



About the year 1666, Newton began to turn his attention to the- 

 consideration of the force which we now call gravity. In that year,, 

 according to the well-known story, he was one day sitting in a garden 

 when he saw an apple fall to the ground, and came to the conclusion' 

 that such a phenomenon could only be the result of the Earth's, 

 attraction. Then it immediately flashed upon him, if the Earthi 

 attracts the apple so that when, by decay, the stalk becomes sufli- 

 cientlv weak, the apple is pulled to the ground, why should not this. 

 same force of attraction extend from the Earth to the Moon ? 



Here the popular story stops, and it is inferred that Newton,, 

 then and there, made his immortal discovery; but in reality it is. 

 just here that the truest interest of the storj' begins. 



The idea of an attractive force like gravity was no new one. 

 Kepler, in his " De Stella Martis," states that every two bodies - 

 of the same kind have the property of attracting each other — thus 

 the Earth attracts a stone and the stone attracts the Earth, but the- 

 attraction of the Earth is much greater than that of the single stone 

 in the proportion of the much greater quantity of matter which it 

 contains. Kepler had discussed the numerous planetary observations 

 of Tycho Brahe, and from these and his own observations had dis- 

 covered his three now well-known laws of planetary motion ; but. 

 apparently, from assuming that the gravitational attractive force 

 between two bodies must vary in direct proportion to the distance- 

 between them, he missed the great generalization which it was left- 

 to Newton to discover. 



Newton, of course, knew that the intensity of the illumination 

 of a surface from a point of light varies inversely as the square of the- 

 distance of that surface from the source of illumination. 



Reasoning by analogv, it seems probable that Xewton would 

 imagine that the force of attraction between two bodies would vary 

 according to the same law, and he worked out a ri'dd mathem.atical: 



