"Lead and helium are no good, helium and bromine equally poor. Lead and bromide are no good--as long as it lasts. Now don't ask me if the combination of the elements interferes. One good detector operates so wonderfully all the time, that a bit of yellow phosphorus is forming phosphorus pentoxid because it is suspended in an atmosphere of pure oxygen." "No apparent determining factors, hey?" "None. You might as well pick out the elements with six-letter names. The periodic chart looks like the scatter-pattern of an open-choke shotgun. Water works fine when it is contained in a glass vessel, but in anything else we know of--no dice." "You seem to have covered a multitude of things," said Dr. Pollard approvingly. "We've had a corps of brilliant, imaginative technicians working on the theory and practise for thirty years. Every one of them has come up with a number of elemental detecting combinations. We're now working on four and five element permutations. "With and without plain and complex electrostatic and magnetic fields--and mixtures of both. We've gone logically as far as we can under a system that demands that we try everything. In each set of permutations, we cover all. You know our motto." Majors finished with a slight laugh. He pointed to the end of the conference room, where, lettered on the wall above the blackboard was-- LEAVE NO TURN UNSTONED! "Where does it come from?" asked Pollard innocently. "Take a fifteen-degree angle from the middle of Boötes. Maybe Arcturus for all we know. Somewhere within fifteen degrees of an arbitrary point up there. A total conic solid angle of thirty degrees will encompass all but wisps of the stuff that filter through once in a year or so." "And the velocity of propagation?" "That's the simplest thing to check. The pulses from the Lawson Radiation follow random patterns. A segment printed along a time-scale can be matched to another segment of the same radiation taken from the other side of the solar system. "It's never perfect enough to do more than approximate the answer, but we've got to get a lot more dispersion than the breadth of the orbit of the planet Pluto before we can detect any time-delay--and if we go too far the synchronization of our test equipment gets more and more difficult. You guess." Pollard thought for a moment. "I can't hope to know all the angles," he said. "This is sufficient until I have to know more about it. Now tell me what might drive a man into instability?" "You tell us," laughed Majors shortly. His laugh was not genuine for he felt the loss of Carroll deeply. "Is there any insoluble dilemma in this at all?" "Not that we know of." Pollard nodded. "People are always confronted with insoluble dilemmas of one sort or another, but most of them could be avoided entirely by a slight change in personal attitude. The man who cannot get a job because of inexperience, and can get no experience for lack of job is in an insoluble dilemma.