Gray Lensman by E.E. Smith — Read

Kinnison did start his flit, but he did not get far. In fact, he did not even reach his squalid room before cold reason told him that the job was only half done -- yes, less than half. He had to give Boskone credit for having brains, and it was not at all likely that even such a comparatively small unit as a planetary headquarters would have only one string to its bow. They certainly would have been forced to install duplicate controls of some sort or other by the trouble they had had after Helmuth's supposedly impregnable Grand Base had been destroyed.

There were other straws pointing the same way. Where had those five strange thought-screened men come from? Bominger hadn't known of them apparently. If that idea was sound, the other headquarters would have a spy ray on the whole thing. Both sides used spy rays freely, of course, and to block them was, ordinarily, worse than to let them come. The enemies' use of the thought-screen was different. They realized that it made it easy for the unknown Lensman to discover their agents, but they were forced to use it because of the deadliness of the supposed mind-ray. Why hadn't he thought of this sooner, and had the whole area blocked off? Too late to cry about it now, though.