but does not detect
the principle of their construction. His previous knowledge of
machines is not sufficient to give him the clue to their explanation.
After an hour of uninterested observation he leaves the hall with a
confused notion of shafts, wheels, cogs, bands, etc., but with no
greater insight into the principles of machinery. Why has one man
learned so much and the other nothing? Because the machinist's
previous experience served as an interpreter and explained these new
contrivances, while the other had no sufficient previous knowledge and
so acquired nothing new. "To him that hath shall be given."

In the act of apperception the old ideas dwelling in the mind are not
to be regarded as dead treasures stored away and only occasionally
drawn out and used by a purposed effort of the memory, but they are
_living forces_ which have the active power of seizing and
appropriating new ideas. Lazarus says they stand "like well-armed men
in the inner stronghold of the mind ready to sally forth and overcome
or make serviceable whatever shows itself at the portals of sense." It
is then through the active aid of familiar ideas that new things find
an introduction to soul life. If old friends go out to meet the
strangers and welcome them, there will be an easy entrance and a quick
adoption into the new home.

But frequently these old friends who stand in the background of our
thoughts must be _awakened_ and called to the front. They must stand
as it were on tiptoe ready to welcome the stranger. For if they lie
asleep in the penetralia of the home the new comers may approach and
pass by for lack of a welcome. It is often necessary, therefore, for
the teacher to revive old impressions, to call up previously acquired
knowledge and to put it in readiness to receive and welcome the new.
The success with which this is done is often the difference between
good and poor teaching.

We might suppose that when two persons look at the same object they
would get the _same impression_, but this is not true at all. Where
one person faints with fright or emotion another sees nothing to be
disturbed at. Two travelers come in sight of an old homestead. To one
it is an object of absorbing interest as the home of his childhood; to
the other it is much like any other old farm house. What is the cause
of this difference? Not the house. It is the same in both cases. It
is remarkable how much color is given to every idea that e