10 PRESIDENT’S ADDRESS. 
planet does not really move in an ellipse but in a kind of hypocycloid, 
and not accurately in that either. 
So it is also with Boyle’s law, and the other simple laws in Physical 
Chemistry. Even Van der Waals’ generalisation of Boyle’s law is only 
a further approximation, 
In most parts of physics simplicity has sooner or later to give place 
to complexity: though certainly I urge that the simple laws were true, 
and are still true, as far as they go, their inaccuracy being only detected 
by further real discovery. The reason they are departed from becomes 
known to us; the law is not really disobeyed, but is modified through the 
action of a known additional cause. Hence it is all in the direction of 
progress. 
It is only fair to quote Poincaré again, now that I am able in the 
main to agree with him— 
_’ Take for instance the laws of reflection. Fresnel established 
them by a simple and attractive theory which experiment seemed 
to confirm. Subsequently, more accurate researches have shown 
that this verification was but approximate; traces of elliptic polari- 
sation were detected everywhere. But it is owing to the first 
approximation that the cause of these anomalies was found, in the 
existence of a transition layer; and all the essentials of Fresnel’s 
theory have remained. We cannot help reflecting that all these 
relations would never have been noted if there had been doubt in 
the first place as to the complexity of the objects they connect. 
Long ago it was said: If Tycho had had instruments ten times as 
precise, we would never have had a Kepler, or a Newton, or 
Astronomy. It is a misfortune for a science to be born too late, 
when the means of observation have become too perfect. That is 
what is happening at this moment with respect to physical 
chemistry: the founders are hampered in their general grasp by 
third and fourth decimal places; happily they are men of robust 
faith. As we get to know the properties of matter better we see 
that continuity reigns... . . It would be difficult to justify [the 
belief in continuity] by apodeictic reasoning, but without [it] all 
science would be impossible.’ 
Here he touches on my own theme, Continuity; for, if we had to 
summarise the main trend of physical controversy at present, I feel 
inclined to urge that it largely turns on the question as to which way 
ultimate victory lies in the fight between Continuity and Discontinuity. 
On the surface of nature at first we see discontinuity; objects 
detached and countable. Then we realise the air and other media, and 
so emphasise continuity and flowing quantities. Then we detect atoms 
