ncil, Stevens began to draw a diagram upon a four-foot sheet of
smooth slate. "You know that your brand of math is over my head like a
circus tent, so we'll let it lie. I'll take your word for it. Steve--if
you're satisfied, it's all x with me."

"I think I can straighten you out a little, by analogy. Here's a rough
sketch of a cylinder, with shade and shadow. You've had descriptive
geometry, of course, and so know that a shadow, being simply a
projection of a material object upon a plane, is a two-dimensional
thing--or rather, a two-dimensional concept. Now take the shade, which
is, of course, this entire figure here, between the cylinder casting
the shadow and the plane of projection. You simply imagine that there
is a point source of light at your point of projection: it isn't really
there. The shade, then, of which I am drawing a picture, has only a
potential existence. You know exactly where it is, you can draw it, you
can define it, compute it, and work with it--but still it doesn't exist;
there is absolutely nothing to differentiate it from any other volume of
air, and it cannot be detected by any physical or mechanical means. If,
however, you place a light at the point of projection, the shade becomes
actual and can be detected optically. By a sufficient stretch of the
imagination, you might compare our beam to that shade. When we turn our
power on, the beam is actual; it is a stream of tangible force, and as
such can be detected electrically. When our switches here are open,
however, it exists only potentially. There is no motion in the ether,
nothing whatever to indicate that a beam had ever actually existed
there. With me?"

"Floundering pretty badly, but I see it after a fashion. You physicists
are peculiar freaks--where we ordinary mortals see actual, solid,
heavy objects, you see only empty space with a few electrons and things
floating around in it; and yet where we see only empty space, you can
see things 'potentially' that may never exist at all. You'll be the
death of me yet, Steve! But I'm wasting a lot of time. What do we do
now?"

"We get busy on the big tube. You might warm up the annealing oven and
melt me that pot of glass, while I get busy on the filament supports,
plate brackets, and so on." Both fell to work with a will, and hours
passed rapidly and almost silently, so intent was each upon his own
tasks.

"All x, Steve." Nadia broke the long silence. "The pyrometer's on the
red, and the o