hat on a table. The hat occults its background, and
thus reconstructs itself.

But when Gracely determined the proper vibrations of his
oscillating current to coincide with all the other material
factors he was using, the final result was before him-real
invisibility. He used a patterned background--a
symmetrically checkered surface, most difficult of all. The
light rays coming from this background passed through the
magnetic field surrounding the invisible colorless cube, and
were bent into a curved path. But their own
color-spectrum--in actuality the color, shape, all the
visible characteristics of the background--was not greatly
altered. The observer saw what actually was behind the
invisible cube: the checkered background, sometimes
slightly distorted, but nevertheless sufficiently clear for
its abnormality to escape notice. Thus the cube's outlines
were not reconstructed; and, in effect, it had vanished.

In practical workings with the X-flyers, no such difficult
test as Gracely's cube and rectangular, symmetrically
patterned background is ever met. The varying background
behind a plane--at rest or flying, and particularly at
night--demands less perfection of background than Gracely's
laboratory conditions. I am informed that an X-flyer can
vaguely be seen--or sensed, rather--from some angles and
under certain and unfavorable conditions of light, and
depending on its line of movement relative to the angle of
observation, and the type and color-lighting of its
background. But under most conditions it represents a very
nearly perfect mechanical invisibility.

There is one aspect of the subject with which I may close
this brief paper. I give it without technical explanation;
it seems to me an amusing angle.

The theory of stereoscopics--the vision of the twin lenses
of the human eyes, set a distance apart to give the
perception of depth, of the third dimension--is in itself a
subject tremendously interesting, and worthy of anyone's
study. I have no space for it here, nor would it be strictly
relevant. I need only state that a two-eyed man sees
partially around an object (by virtue of the different
angles from which each of his eyes gaze at it) and thus sees
a trifle more of the background than would otherwise be t