From "The Defiant Agents" by Andre Norton: "But the young Apache stumbled, rebounding back as if he had run into an unseen wall—when his knife was still six inches away from the other. Lupe cried out, shook under a second impact as the Russian fired an automatic with his other hand. ... But even as Lupe's knife had never touched flesh, so was the rock deflected; the Russian was covered by some protective field. This was certainly nothing the Apaches had seen before. Nolan's whistle summoned them to draw back."