THE PRESIDENTIAL ADDRESS. 23 
ment during this epoch of the technical methods of attack, for problems 
that at one time seemed unattackable are now seen to be likely to fall 
before the new methods. In the main, the epoch under consideration 
has been an age of experiment, where the experimenter has been the 
pioneer in the attack on new problems. At the same time, it has 
been also an age of bold ideas in theory, as the Quantum Theory and 
the Theory of Relativity so well illustrate. 
I feel it is a great privilege to have witnessed this period, which 
may almost be termed the Renaissance of Physics. It has been of 
extraordinary intellectual interest to watch the gradual unfolding of 
new ideas and the ever-changing methods of attack on difficult problems. 
It has been of great interest, too, to note the comparative simplicity 
of the ideas that have ultimately emerged. For example, no one could 
have anticipated that the general relation between the elements would 
prove to be of so simple a character as we now believe it to be. It is 
an illustration of the fact that Nature appears to work in a simple way, 
and that the more fundamental the problem often simpler are the con- 
ceptions needed for its explanation. The rapidity and certitude of the 
advance in this epoch have largely depended on the fact that it has been 
possible to devise experiments so that few variables were involved. For 
example, the study of the structure of the atom has been much facili- 
tated by the possibility of examining the effects due to a single atom 
of matter, or, as in radioactivity or X-rays, of studying processes going 
on in the individual atom which were quite uninfluenced by external 
- conditions. 
. In watching the rapidity of this tide of advance in physics I have 
_ become more and more impressed by the power of the scientific method 
_ of extending our knowledge of Nature. Experiment, directed by the 
_ disciplined imagination either of an individual, or still better, of a 
_ group of individuals of varied mental outlook, is able to achieve results 
which far transcend the imagination alone of the greatest natural philo- 
sopher. Experiment without imagination, or imagination without 
recourse to experiment, can accomplish little, but, for effective progress, 
a happy blend of these two powers is necessary. The unknown appears 
as a dense mist before the eyes of men. In penetrating this obscurity 
we cannot invoke the aid of supermen, but must depend on the com- 
bined efforts of a number of adequately trained ordinary men of scientific 
imagination. Each in his own special field of inquiry is enabled by 
the scientific method to penetrate a short distance, and his work reacts 
upon and influences the whole body of other workers. From time to 
time there arises an illuminating conception, based on accumulated 
knowledge, which lights up a large region and shows the connection 
between these individual efforts, so that a general advance follows. The 
attack begins anew on a wider front, and often with improved technical 
