THE PRESIDENTIAL ADDRESS 3 



observation, even when at first sight they appeared to be hopelessly 

 beyond it. 



For example, at the time of Lord Balfour's address some who 

 were regarded as leaders of scientific thought still urged that 

 the conception of atoms was not to be taken literally. We now 

 count the atoms by direct methods. We see the electrometer 

 needle give a kick and we say, ' There goes an atom.' Or we 

 see the path of an individual atom marked out by a cloud track 

 and we see v/here it was abruptly bent by a violent collision with 

 another atom. 



Again, the theory of radioactive decomposition put forward by 

 Rutherford, however cogent it may have seemed and did seem to 

 those who were well acquainted with the evidence, was originally 

 based on indirect inferences about quantities of matter far too small 

 to be weighed on the most delicate balance. Chemists were naturally 

 inclined to feel some reserve ; but in due course the theory led to a 

 conclusion which could be tested by methods in which they had 

 confidence — the conclusion, namely, that lead contained in old 

 uranium minerals ought to have a lower atomic weight than ordinary 

 lead and in all probability to be lighter, and on trying this out it 

 proved to be so. More recently we have the discovery of heavy 

 hydrogen with twice the density of ordinary hydrogen and heavy 

 water which is the source of it. 



Lastly, the conclusion that ordinary matter is not really space- 

 filling has been illustrated by the discovery that certain stars have a 

 density which is a fabulous multiple of the density of terrestrial 

 matter. Although this is in some sense a deduction as distinguished 

 from an observation, yet the steps required in the deduction are 

 elementary ones entirely within the domain of the older physics. 



This and many other points of view have seemed at first sight to 

 contradict the direct indication of our senses. But it was not really 

 so. They were obtained and could only be obtained by sense 

 indications rightly interpreted. As in the passage from Lord Balfour 

 already quoted the senses were not primarily developed for purposes 

 of research, and we have in large measure to adapt them to that 

 purpose by the use of artificial auxiliaries. The result of doing so 

 is often to reveal a world which to the unaided senses seems 

 paradoxical. 



I have chosen for the main subject of this address a survey of some 

 of the ways in which such adaptations have been made. I shall 

 naturally try to interest you by dwelling most on aspects of the subject 

 that have some novelty ; but apart from these there is much to be 

 gleaned of historical interest, and when tempted I shall not hesitate 

 to digress a little from methods and say something about results. 



I shall begin with a glance at the mechanism of the human eye, 



