"Take a concrete example--the formula I used Sunday night. It shows signs of recent revision. For instance, what did the original, unrevised formula have in place of the phonograph needle?" "A willow whistle of a certain shape, which had been blown only once," the voice told him. "And the platinum or iridium?" "The original formula mentioned silver, but a heavier metal serves better. Lead, however, proved altogether ineffective. I tried it once. It was apparently too unlike silver in other respects." "Precisely. Trial-and-error experimentation. I have a modern substitute for the flannel wrapping which may prove more effective. Moreover, in the absence of thorough investigation, we cannot be sure that all the ingredients of a magic formula are essential in making it work. A comparison of the magic formulas of different countries and peoples would be helpful in this respect. It would show which ingredients are common to all formulas and therefore presumably essential, and which are not essential. I have in mind a method for making such a comparison." * * * * * There was a discreet knock at the door. Norman spoke a few words, and the figure drew down its veil and turned toward the window, as if staring stolidly at the passing fields. Then he opened the door. It was lunch, as long in coming as breakfast had been. And there was a new face--coffee-colored instead of ebony. Evidently the first waiter, who had shown growing nervousness in his previous trips to the compartment, had decided to sacrifice the tip and send someone else. With a mixture of curiosity and impatience, Norman waited for the reactions of the new-comer. He almost felt able to predict them. First a very quick inquisitive glance past him at the seated figure--Norman guessed they had become the major mystery of the train. Then a longer, sideways glance while setting up the folding table, ending with the eyes getting very wide; he could almost feel the coffee-colored flesh crawl. Only hurried, almost unwilling glances after that, with a growing uneasiness manifested in clumsy handling of the dishes and glassware. Then a too-pleasant smile and a hasty departure. Only once Norman interfered--to place the knives and forks so they lay at right angles to their usual position.