One was Roscoe Gilkey Dickinson. He had got his doctorate in physical chemistry — x-ray crystallography — in 1920. He was the first person to get a Ph.D. from California Institute of Technology. And he was continuing with x-ray crystallography, determining the structure of inorganic crystals. After I had accepted appointment as a graduate student at California Institute of Technology, A.A. Noyes, the head of the division of chemistry and chemical engineering, wrote to say that he had decided I should work with Roscoe Dickinson on determining the structure of crystals. This was really extremely fortunate for me, in my opinion. I don’t think that there was any field that was more suited to my interests, and I don’t really know why Dr. Noyes selected me out of eight or ten new graduate students to do x-ray crystallography. Dickinson had a remarkable mind. He was a very careful investigator and thinker, a very logical thinker. When he was teaching me x-ray crystallography, he also taught me to ask at each stage in the argument, “What assumptions are being made? How reliable is the conclusion that you draw? What chance is there that one of the assumptions you have made is not correct?” “You should recognize,” he said to me, “that there is in almost every investigation a lack of complete rigor. You should understand just how reliable the arguments are that you are presenting.