concentration on the final problem of finding the
right carbon filament and determining the
proper degree of vacuum in the inclosing
bulb. Months of experiment and research
had gone before; eighteen hours a day in the
laboratory had been no uncommon thing for
the inventor and his assistants, but in the last
strenuous grapple with success his own physical
and mental powers were alone equal to the
strain. Not once during the two days and
nights did he rest or sleep or take his attention

from the successive tests which led up to the
assembling of the lamp which lights the world's
work and play.

The steel blade that is used seems to last as
long as the one which is allowed to lie idle.
The wearing out in the one case does not seem
to be more destructive than the rusting out
in the other.

We have a choice between wearing out and
rusting out. Most of us unwittingly have
chosen the rusting process.

This, indeed, may be said to be Edison's
regular method of work, as it is the method of
many other men who have accomplished great
things in science and industry. Both mind