ity locator, a sensitive short-wave directional receiver, that
would permit the operator to direct his rays at invisible targets. The
ships themselves could not be made invisible, since they depended in
their very principle on the absorption of light-energy. If the walls of
every part of the ship were perfectly transparent, they could absorb no
energy at all, and they would still be plainly visible--even more so
than before! They must remain visible, but they could also force the
enemy to remain visible.

Each ten-man ship carried an old-fashioned cannon that was equipped to
hurl cannisters carrying the luminous paint. They decided that these
would have advantages, even if the invaders did not use invisibility,
for in space a ship is visible only because it reflects or emits light.
For this reason the ships were not equipped with any portholes except in
the pilot room and at the observation posts. No light could escape. To
reduce the reflection to the absolute minimum, the ships had each been
painted with a 99% absorptive black. In space they would be exceedingly
difficult targets.

The heating effect of the sun on the black pigment when near the great
star was rather disagreeably intense, and to cool the speedsters they
had installed molecular director power units, which absorbed the heat
and used the energy to drive the ship. Heaters offset the radiation loss
of the black surface when too far from the sun.

Each of the speedsters was equipped with a small machine-gun shooting
luminous paint bullets. One of these, landing on another craft, made it
visible for at least two hours, and since they could cover an area of
about thirty feet, they were decidedly effective.

It was found that ray practice was rather complicated. The government
had ranges set up in great mountain districts away from any valuable
property, but they soon found that spatial warplay could not be carried
on on Earth. The rays very quickly demolished the targets, and in a
short time made good progress toward demolishing the mountains as well.
The problem was solved by using the barren surface of the moon and the
asteroid belt beyond Mars as a proving ground.

The ships were sent out in squadrons as fast as they could be finished
and the men could be brought together and trained. They were
establishing a great shield of ships across all that section of the
system whence the Nigrans had appeared, and they hoped to intercept the
next attack before i