4.30 REPORT—1904. 
it altogether. The world remains a more wonderful place than ever; we may be 
sure that it abounds in riches not yet dreamed of; and, although we cannot hope 
ever to explore its innermost recesses, we may be confident that it will supply 
tasks in abundance for the scientific mind for ages to come. 
One significant result of the modern tendency is that we no longer with the 
same obstinacy demand a mechanical explanation of the phenomena of Light and 
Electricity, especially since it has been made clear that if one mechanical explana- 
tion is possible, there will be an infinity of others. Some minds, indeed, revelling 
in their new-found freedom, have attempted to disestablish ordinary or ‘ vulgar’ 
matter altogether. I may refer to a certain treatise which, by some accident, 
does not bear its proper title of ‘ AXther and zo Matter,’ and to the elaborate 
investigations of Professor Osborne Reynolds, which present the same peculiarity, 
although the basis is different. Speculations of this nature have, however, been 
so recently and (if I may say it) so brilliantly dealt with by Professor Poynting 
before this Section that there is little excuse for dwelling further on them now. 
I will only advert to the question whether, as some suggest, physical science 
should definitely abandon the attempt to construct mechanical theories in the 
older sense. The question would appear to be very similar to this, whether we 
should abandon the use of graphical methods in analysis? In either csae we run 
the risk of introducing extraneous elements, possibly of a misleading character ; 
but the gain in vividness of perception and in suggestiveness is so great that we 
are not likely to forego it, by excess of prudence, in one case more than in the 
other. 
We have travelled some distance from Stokes and the mathematical physics 
of half a century ago. May I add a few observations which might perhaps have 
claimed his sympathy? They are in substance anything but new, although I do 
not find them easy. to express. We have most of us frankly adopted the empirical 
attitude in physical science ; it has justified itself abundantly in the past, and has 
more and more forced itself upon us. We have given up the notion of causation, 
except asa convenient phrase; what were once called laws of Nature are now 
simply rules by which we can tell more or less accurately what will be the conse- 
quences of a given state of things. We cannot help asking, How is it that such 
rules are possible? A rule is invented in the first instance to sum up in a com- 
pact form a number of past experiences; but we apply it with little hesitation, 
and generally with success, to the prediction of new and sometimes strange ones. 
Thus the law of gravitation indicates the existence of Neptune; and Fresnel’s 
wave-surface gives us the quite unsuspected phenomenon of conical refraction. 
Why does Nature make a point of honouring our cheques in this manner ; or, to 
put the matter ina more dignified form, how comes it that, in the words of 
Schiller, 
‘Mit dem Genius steht die Natur im ewigen Bunde, 
Was der eine verspricht, leistet die andre gewiss’ ? 
The question is as old as science, and the modern tendencies with which we have 
been occupied have only added point to it. It is plain that physical science as 
such has no answer; its policy indeed has been to retreat from a territory which 
it could not securely occupy. We are told in some quarters that it is vain to 
lock for an answer anywhere. But the mind of man is not wholly given over 
to physical science, and will not be content for ever to leave the question alone. 
It will persist in its obstinate questionings, and, however hopeless the attempt 
to unravel the mystery may be deemed, physical science, powerless to assist, has 
no right to condemn it. 
I would like, in conclusion, to read to you a characteristic passage from 
that Address of Stokes in 1862 which has formed the starting-point of this dis- 
course :—- 
‘In this Section, more perhaps than in any other, we have frequently to deal 
with subjects of a very abstract character, which in many cases cau be mastered 
1 Applied by Sir J. Herschel to the discovery of Neptune. 
