above it.

A complete apparatus may be "set" for "penetrative," "distance" and
"normal vision."

In the first, which one would use to look through the forest screen from
the air, or in examining the interior of a Han ship or any opaque
structure, the _glow-spot_ is brought low, at only a tiny angle above
the vision line, and the shield, of course, must be very carefully
adjusted.

"Distance" setting would be used, for instance, in surveying a valley
beyond a hill or mountain; the _glow-spot_ is thrown high to illuminate
the entire scene.

In the "normal" setting the _foco_ rays are brought together close
overhead, and illuminate the scene just as a lamp of super brilliancy
would in the same position.

For phonic communication a spherical sending battery is a ball of
metultron, surrounded by an insulating shell of inertron, and this in
turn by a spherical shell of katultron, from which the current radiates
in every direction, tuning being accomplished by frequency of
intermissions, with audio-frequency modulation. The receiving battery
has a core pole of katultron and an outer shell of metultron. The
receiving battery, of course, picks up all frequencies, the undesired
ones being tuned out in detection.

Tuning, however, is only a convenience for privacy and elimination of
interference in ultrophonic communication. It is not involved as a
necessity, for untuned currents may be broadcast at voice-controlled
frequencies, directly and without any carrier wave.

To use plate batteries or single center-line batteries for phonic
communication would require absolutely accurate directional aligning of
sender and receiver, a very great practical difficulty, except when
sender and receiver are relatively close and mutually visible.

* * * * *

This, however, is the regular system used in the Inter-Gang network for
official communication. The senders and receivers used in this system
are set only with the greatest difficulty, and by the aid of the finest
laboratory apparatus, but once set, they are permanently locked in
position at the stations, and barring earthquakes or insecure
foundations, need no subsequent adjustment. Accuracy of alignment
permits beam paths no thicker than the old lead pencils I used to use in
the Twentieth Century.

The non-interference of such communication lines, and the difficulty of
cutting in on them from any point except immediately adjacent to the
sender