Sixth Column by Robert A. Heinlein

two
    rabbits apart with it, or you could tell a rabbit from a dog, simply by the
    spectra of their hemoglobins.
    "This Dr. Fox tried to do the same thing with humans, but it didn't workno distinguishable difference in wavelengths. But Calhoun and Wilkie have
    rigged a spectroscope for the spectrum Ledbetter was playing with, and it
    shows clearly separate wavelengths for each sample of human blood.
    Conversely, if they set up a tuned Ledbetter projector and start running down
    or up the scale, when they come to your individual, unique frequency, your
    red blood cells start absorbing energy, the hemoglobins protein breaks down
    and-Spung!-you're dead. I'm standing right beside you and I'm not even hurt;
    they haven't come to my frequency. Now Brooks has an idea that these
    frequencies come by groups according to races. He thinks they can tune it to
    discriminate by races, to knock over all the Asiatics in a group and not touch
    the white men, and vice versa."
    Thomas shivered. "Whew! That would be a weapon. "
    "Yes, it would. It's just on paper so far, but they want to test it on Mitsui.
    As I gather what they intended to do, they don't intend to kill him, but it's
    bound to be dangerous as all hell to Mitsui."
    "Frank won't mind chancing it," Thomas commented.
    "No, I don't suppose he would." It seemed to Ardmore that it would
    probably be a favor to Mitsui to give him a clean, painless death in the
    laboratory. "Now about another matter. It seems to me we ought to be able to
    work up a sort of permanent secret service, using your hobo pals and their
    sources of information. Let's talk about it."
    Ardmore gained a few days' respite in which to consider further the
    problem of military use of the weapons at his disposal while the research
    staff tested their theories concerning the interrelation between racial types
    and the improved Ledbetter effect. The respite did him no good. He had a
    powerful weapon, yes; in fact, many powerful weapons, for it seemed that the
    new principles they had tapped had fully as protean possibilities as electricity.
    It seemed extremely likely that if the United States defense forces had had,
    one year earlier, the tools now available in the Citadel, the United States
    would never have fallen.
    But six men cannot whip an empire-not by brute force. The emperor
    could, if necessary, expend six million men to defeat six. The hordes of the
    empire could come at them barehanded and win, move over them as an
    avalanche moves, until they were buried under a mountain of dead flesh.
    Ardmore had to have an army to fight with his wonderful new weapons.
    The question was: how to recruit and train such an army?
    Certain it was that the PanAsians would not hold still while he went into
    the highways and byways and got his forces together. The thoroughness with
    which they had organized police surveillance of the entire population made it
    evident that they were acutely aware of the danger of revolution and would
    stamp out any such activity before it could possibly reach proportions
    dangerous to them.
    There remained one clandestine group, the hobos.
    He consulted with Thomas as to the possibility of organizing them for
    military purposes. Thomas shook his head at the idea.
    "You can't understand the hobo temperament, Chief. There is not one in
    a hundred who could be depended on to observe the strict self -discipline
    necessary for such an enterprise. Suppose you were able to arm all of them
    with projectors -I don't say that is possible, but suppose you could-you still
    would not have an army; you would simply have an undisciplined rabble."
    "Wouldn't they fight?"
    "Oh, sure, they'd fight. They'd fight as individuals, and they would do
    quite a bit of slaughter until some flatface caught them off guard and winged
    them. "
    "I wonder if we can depend on them as sources of information. "
    "That's another matter. Most of the road kids won't have any idea that
    they are being used to obtain military