18 THE PRESIDENTIAL ADDRESS. 



Notliing in the progress of science, and more particularly of modern 

 science, is so impressive as the growing appreciation of the immensity of 

 what awaits discovery, and the contrasted feebleness of our ability to 

 put into words even so much as we already dimly apprehend. Let me 

 take an example from the world of the physical sciences. There is a 

 problem of which the minds of physicists have been full in recent years. 

 The nineteenth-century theory of radiation asks us to look on light as a 

 series of waves in an all-pervading ether. The theory has been marvel- 

 lously successful, and the great advances of nineteenth-century physics 

 were largely based upon it. It can satisfy the fundamental test of all 

 theories, for it can predict the occurrence of effects which can be tested 

 by experiment and found to be correct. There is no question of its truth 

 in the ordinary sense. 



In the last twenty or thirty years a vast new field of optical research 

 has been opened up, and among the curious things we have found is the 

 fact that light has the properties of a stream of very minute particles. 

 Only on that hypothesis can many experimental facts be explained. A 

 wave theory is of no use in the newer field. How are the two views to be 

 reconciled ? How can anything be at once a wave and a particle ? I 

 do not believe that I am unjust to any existing thinker if I say that no 

 one yet has bridged the gap. Some of you who were present at the 

 Liverpool meeting may remember that Bohr — one of the leading physicists 

 of the world — doubted if the human mind was yet sufficiently developed 

 to the stage in which it would be able to grasp the whole explanation. 

 It may be a step forward to say, as we have been saying vaguely for some 

 years, that both theories are true, that there are corpuscles and there 

 are waves and that the former are actually responsible for the transference 

 of energy in light and heat, and for making us see ; while the latter guide 

 the former on their way. This is going back to Newton, who expressed 

 ideas of this kind in his ' Opticks,' though he was careful to add that they 

 were no more than a suggestion. 



We are here face to face with a strange problem. We know that there 

 must be a reconcilement of our contradictory experiments ; it is surely 

 our conceptions of the truth which are at faidt, though each conception 

 seems valid and proved. There must be a truth which is greater than 

 any of our descriptions of it. Here is an actual case where the human 

 mind is brought face to face with its own defects. What can we do ? 

 What do we do ? As physicists we use either hypothesis according to 

 the range of experiences that we wish to consider. To repeat a phrase 



