10 R. THRELFALL. 
the principle was discovered by Helmholtz—but for some reason 
on other his essay seems to have had greater persuasive powers 
than the work of other and earlier labourers in the same field. 
The reason of this was in my opinion—and I offer it with the 
utmost diffidence and every possible reservation—that he was 
the first to recognise that change of the potential function is in 
a certain aspect the measure of a distinct entity, viz.—that 
knowable, but unknown thing potential energy—thus christened 
by Rankine. But Helmholtz’s claims to immortality, rest on a 
broader basis than is afforded by any one discovery. He laid the 
foundation of that science of Bacteriology, which promises to 
assist mankind in coping with his environment more powerfully 
than any discovery since the steam engine. It is understood 
that Ophthalmic surgery is based largely on his work on Physio- 
logical optics. Electrical theory is indebted to him not only for 
many notable discoveries but for the training and inspiring of 
Hertz. To his mathematical powers we owe the theory of vortex 
motion, and many important advances in Hydrodynamics. His 
great “ Tonenempfindungen ” in which he appears as Physicist, 
Physiologist, Anatomist, and Psychologist might well have been 
the life work of an ordinary mortal, to him it was a mere 
episode. There was a time when we hoped that he had built the 
bridge leading from the physical and mathematical theory of 
sound to the doctrine of harmony, but fate has been against 
this courageous and ingenious effort, and at the present time it is 
very doubtful if it can be said to have materially advanced the 
position. The energies of Helmholtz’s later life were partly 
absorbed in the councils of his country ; and partly devoted to 
the organisation of the great testing and standardising Laboratory 
at Charlottenburg. | 
Though it is generally desirable to subdue the personal element 
as far as possible ; on an occasion of this kind I feel that perhaps 
the members present might like to know something of the 
personality of this, probably the most versatile and learned philoso- 
pher that ever lived. In the winter of 1888 I presented myself at 
