“That sounds reasonable,” acceded Markham. “But where did Dillard get his fantastic ideas for the murders?”
“The Mother-Goose motif was probably suggested to him when he heard Arnesson jestingly tell Robin to beware of an arrow from Sperling’s bow. He saw in that remark a means of venting his hatred against the man who had made it; and he bided his time. The opportunity to stage the crime came shortly after. When he saw Sperling pass up the street that morning, he knew that Robin was alone in the archery-room. So he went below, engaged Robin in conversation, struck him over the head, drove a shaft into his heart, and shoved him out on the range. He then wiped up the blood, destroyed the cloth, posted his notes at the corner, put one in the house letter-box, returned to the library, and called up this office. One unforeseen factor cropped up, however:—Pyne was in Arnesson’s room when the professor said he went out on the balcony. But no harm came of it, for though Pyne knew something was amiss when he caught the professor lying, he certainly didn’t suspect the old gentleman of being a murderer. The crime was a decided success.”
“Still and all,” put in Heath, “you guessed that Robin hadn’t been shot with a bow and arrow.”
“Yes. I saw from the battered condition of the nock of the arrow that it had been hammered into Robin’s body; and I concluded therefore that the chap had been killed indoors, after having first been stunned with a blow on the head. That was why I assumed that the bow had been thrown to the range from the window,—I didn’t know then that the professor was guilty. The bow of course was never on the range.—But the evidence on which I based my deductions cannot be held as an error or oversight on the professor’s part. As long as his Mother-Goose joke was accomplished, the rest didn’t matter to him.”
“What instrument do you think he used?” Markham put the question.
“His walking stick, most likely. You may have noticed it has an enormous gold knob perfectly constructed as a lethal weapon.[41] Incidentally, I’m inclined to think he exaggerated his gout to attract sympathy and to shunt any possible suspicion from himself.”
“And the suggestion for the Sprigg murder?”
“After Robin’s death he may have deliberately looked about for Mother-Goose material for another crime. In any event, Sprigg visited the house the Thursday night preceding the shooting; and it was at that time, I imagine, that the idea was born. On the day chosen for the gruesome business he rose early and dressed, waited for Pyne’s knock at half past seven, answered it, and then went to the park—probably through the archery-room and by way of the alley. Sprigg’s habit of taking daily morning walks may have been casually mentioned by Arnesson, or even by the lad himself.”
“But how do you explain the tensor formula?”
“The professor had heard Arnesson talking to Sprigg about it a few nights before; and I think he placed it under the body to call attention—through association—to Arnesson. Moreover, that particular formula subtly expressed the psychological impulse beneath the crimes. The Riemann-Christoffel tensor is a statement of the infinity of space—the negation of infinitesimal human life on this earth; and subconsciously it no doubt satisfied the professor’s perverted sense of humor, giving added homogeneity to his monstrous conception. The moment I saw it I sensed its sinister significance; and it substantiated my theory that the Bishop murders were the acts of a mathematician whose values had become abstract and incommensurable.”