s out among a city
full of buildings and lights up, while the remainder are left in the
semilight or in darkness.

ATTENTION MEASURES MENTAL EFFICIENCY.--In a state of attention the mind
may be likened to the rays of the sun which have been passed through a
burning glass. You may let all the rays which can pass through your
window pane fall hour after hour upon the paper lying on your desk, and
no marked effects follow. But let the same amount of sunlight be passed
through a lens and converged to a point the size of your pencil point,
and the paper will at once burst into flame. What the diffused rays
could not do in hours or in ages is now accomplished in seconds.
Likewise the mind, allowed to scatter over many objects, can accomplish
but little. We may sit and dream away an hour or a day over a page or a
problem without securing results. But let us call in our wits from their
wool-gathering and "buckle down to it" with all our might, withdrawing
our thoughts from everything else but this _one thing_, and
concentrating our mind on it. More can now be accomplished in minutes
than before in hours. Nay, _things which could not be accomplished at
all before_ now become possible.

Again, the mind may be compared to a steam engine which is constructed
to run at a certain pressure of steam, say one hundred and fifty pounds
to the square inch of boiler surface. Once I ran such an engine; and
well I remember a morning during my early apprenticeship when the
foreman called for power to run some of the lighter machinery, while my
steam gauge registered but seventy-five pounds. "Surely," I thought, "if
one hundred and fifty pounds will run all this machinery, seventy-five
pounds should run half of it," so I opened the valve. But the powerful
engine could do but little more than turn its own wheels, and refused
to do the required work. Not until the pressure had risen above one
hundred pounds could the engine perform half the work which it could at
one hundred and fifty pounds. And so with our mind. If it is meant to do
its best work under a certain degree of concentration, it cannot in a
given time do half the work with half the attention. Further, there will
be much _which it cannot do at all_ unless working under full pressure.
We shall not be overstating the case if we say that as attention
increases in arithmetical ratio, mental efficiency increases in
geometrical ratio. It is in large measure a difference in the power of
at