_Foco_ batteries are twin batteries, arranged to shoot a positive and a negative beam in the same direction. When these beams are made intermittent at light frequencies (though they are not light waves, nor of the same order as light waves) and are brought together, or focussed, at a given spot, the space in which they cross radiates alternating ultronic current in every direction. This radiated _ultralight_ acts like true light so long as the crossing beams vibrate at light frequencies, except in three respects: first, it is not visible to the eye; second, its "color" is exclusively dependent on the frequency of the _foco_ beams, which determine the frequency of the alternating radiation. Material surfaces, it would appear, reflect them all in equal value, and the color of the resultant picture depends on the color of the _foco_ frequencies. By altering these, a reddish, yellowish or bluish picture may be seen. In actual practice an orthochromatic mixture of frequencies is used to give a black, gray and white picture. The third difference is this: rays pulsating in line toward any ultron object connected with the rear plates of the twin batteries through rectifiers cannot be reflected by material objects, for it appears they are subject to a kind of "pull" which draws them straight through material objects, which in a sense are "magnetized" and while in this state offer no resistance. Ultron, when so connected with battery terminals, glows with true light under the impact of _ultralight_, and if in the form of a lens or set of lenses, may be made to deliver a picture in any telescopic degree desired. * * * * * The essential parts of an ultroscope, then, are twin batteries with focal control and frequency control; an ultron shield, battery connected and adjustable, to intercept the direct rays from the "_glow-spot_," with an ordinary light-shield between it and the lens; and the lens itself, battery connected and with more or less telescopic elaboration. To look through a substance at an object, one has only to focus the _glow-spot_ beyond the substance but on the near side of the object and slightly 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.