you, as teachers, I would say, mere book learning in physical science
is a sham and a delusion--what you teach, unless you wish to be
impostors, that you must first know; and real knowledge in science
means personal acquaintance with the facts, be they few or many.*

[footnote] *It has been suggested to me that these words may
be taken to imply a discouragement on my part of any sort
of scientific instruction which does not give an
acquaintance with the facts at first hand. But this is not
my meaning. The ideal of scientific teaching is, no doubt,
a system by which the scholar sees every fact for himself,
and the teacher supplies only the explanations.
Circumstances, however, do not often allow of the
attainment of that ideal, and we must put up with the next
best system--one in which the scholar takes a good deal on
trust from a teacher, who, knowing the facts by his own
knowledge, can describe them with so much vividness as to
enable his audience to form competent ideas concerning
them. The system which I repudiate is that which allows
teachers who have not come into direct contact with the
leading facts of a science to pass their second-hand
information on. The scientific virus, like vaccine lymph,
if passed through too long a succession of organisms, will
lose all its effect in protecting the young against the
intellectual epidemics to which they are exposed.