Late that autumn of 1925, Robert did something so stupid that it seemed calculated to prove that his emotional distress was overwhelming him. Consumed by feelings of inadequacy and intense jealousy, he “poisoned” an apple with chemicals from the laboratory and left it on Blackett’s desk. Jeffries Wyman later said, “Whether or not this was an imaginary apple, or a real apple, whatever it was, it was an act of jealousy.” Fortunately, Blackett did not eat the apple; but university officials somehow were informed of the incident. As Robert himself confessed to Fergusson two months later, “He had kind of poisoned the head steward. It seemed incredible, but that was what he said. And he had actually used cyanide or something somewhere. And fortunately the tutor discovered it. Of course there was hell to pay with Cambridge.” If the alleged “poison” was potentially lethal, what Robert had done amounted to attempted murder. But this seems improbable, given what happened next. More likely, Robert had laced the apple with something that merely would have made Blackett sick; but this was still a serious matter—and grounds for expulsion.
As Robert’s parents were still visiting Cambridge, the university authorities immediately informed them of what had happened. Julius Oppenheimer frantically—and successfully—lobbied the university not to press criminal charges. After protracted negotiations, it was agreed that Robert would be put on probation and have regular sessions with a prominent Harley Street psychiatrist in London. As Robert’s old Ethical Culture School mentor, Herbert Smith, put it, “He was retained at Cambridge for a while only on condition that he had periodic interviews with a psychiatrist.”