XXII
335
HERMANN VON HELMHOLTZ
living eye. Beyond the doubtful and unreliable feelings of
the patient there were no means of diagnosing the disease or
determining the defects in refraction. Before any cure was
possible it was absolutely necessary for the surgeon to acquire
an accurate knowledge of the disease; and this, in the
majority of cases, was only attainable after the invention of
this simple instrument. Ophthalmic surgery rapidly rose to
its present high level. Who can say how many thousands
who have recovered their sight owe it to our investigator-to
him personally, although they are unconscious of this and
think that their thanks are simply due to the surgeon who
has treated them! The invention of the ophthalmoscope is
like vaccination against smallpox, the antiseptic treatment of
wounds, or the sterilisation of children's food-one of those
great gifts which enrich all without impoverishing any, one of
those advances which are gratefully acknowledged everywhere
by all men, and which keep alive in us the belief that there is
such a thing as progress.
Equally powerful as a protection against blindness on the
intellectual side are the advances which physiology owes to
Helmholtz, although their value may not be so easily or
generally recognised. Here we may remind the reader in
passing that he was the first to measure the speed with
which sensation and volition travel along the nerves: this
would have sufficed to establish the fame of any other man,
but it is not this that we now have in mind. His chief
investigation in this science, the work of his mature years, is
the development of the physiology of the senses, especially of
sight and hearing. Within our consciousness we find an
inner intellectual world of conceptions and ideas outside
our consciousness there lies the cold and alien world of
actual things. Between the two stretches the narrow border-
land of the senses. No communication between the two
worlds is possible excepting across this narrow strip. No
change in the external world can make itself felt by us unless
it acts upon a sense-organ and borrows form and colour from
this organ.
In the external world we can conceive no causes
for our changing feelings until we have, however reluctantly,
assigned to it sensible attributes. For a proper understand-
ing of ourselves and of the world it is of the highest import-