“Take the nose, for example,” explained Kennedy.  “There are only three kinds, as Bertillon calls them—­convex, straight, and concave.  A detective, we will say, is sent out after a man with a concave nose or, as in this case a woman with a straight nose.  Thus he is freed from the necessity of taking a second glance at two-thirds of the women, roughly, that he meets—­that is, theoretically.  He passes by all with convex and concave noses.

“There are four classes of ears—­triangular, square, oval, and round, as they may be called.  Having narrowed his search to women with straight noses, the detective needs to concern himself with only one-fourth of the women with straight noses.  Having come down to women with straight noses and, say, oval ears, he will eliminate all those that do not have the mouth, lips, chin, eyes, forehead, and so on that have been given him.  Besides that, there are other striking differences in noses and ears that make his work much easier than you would imagine, once he has been trained to observe such things quickly.”

“It sounds all right,” I agreed haltingly.

“It is all right, too,” he argued warmly.  “The proof of it is its use in Paris and other cities abroad and the fact that it has been imported here to New York in the Police Department and has been used by the Government.  I could tell you many interesting stories about how it has succeeded where photographs would have failed.”

I had been reading over the description again and trying to apply it.

“For instance,” Craig resumed thoughtfully.  “I believe that this woman is a mulatto, but that is a long way from proving it.  Still, I hope that by using the portrait parle and other things we may be able to draw the loose threads together into a net that will catch her—­providing, of course, that she ought to be caught.”

He had finished making copies of the portrait parle and had called for a cheque for the lunch.

“So you see,” he concluded, “this is without any doubt the woman we saw at the Futurist, whom Miss Kendall followed to Madame Margot’s Beauty Shop, two doors down.”

Kennedy handed a copy to Miss Kendall.

“Using that and whatever other means you may have, Miss Kendall,” he said, “I wish that you would try to find this woman and all you can about her.  Walter, take this other copy and see Carton.  I think he has a county detective who knows the system.  I shall spend the rest of the day getting in touch with the Federal authorities in this city and in Washington trying to find out whether they know anything about her.”

We left the Montmartre with as much care as we had entered and seemingly without having yet aroused any suspicion.  The rest of the day was spent in setting to work those whom we felt we could trust to use the portrait parle to locate the mysterious dark-haired Marie who seemed to cross our trail at every turn, yet who proved so elusive.