Kennedy said nothing as he finally tied up the little packet of letters and locked it in his safe. He was not given to hasty generalizations; neither was he one who clung doggedly to a preconceived theory.
It was still early in the afternoon. Craig and I decided to drop into the museum again in order to see Doctor Bernardo. He was not there and we sat down to wait.
Just then the letter box in the door clicked. It was the postman on his rounds. Kennedy walked over and picked up the letter.
The postmark bore the words, “Mexico City,” and a date somewhat later than that on which Northrop had left Vera Cruz. In the lower corner, underscored, were the words, “Personal—Urgent.”
“I’d like to know what is in that,” remarked Craig, turning it over and over.
He appeared to be considering something, for he rose suddenly and shoved the letter into his pocket.
I followed, and a few moments later, across the campus in his laboratory, he was working quickly over an X-ray apparatus. He had placed the letter in it.
“These are what are known as ‘low’ tubes,” he explained. “They give out ‘soft rays.’” He continued to work for a few moments, then handed me the letter.
“Now, Walter,” he said, “if you will just hurry back to the museum and replace that letter, I think I will have something that will astonish you—though whether it will have any bearing on the case, remains to be seen.”
“What is it?” I asked, a few minutes later, when I had rejoined him, after returning the letter. He was poring intently over what looked like a negative.