The best way to convince him is to show him that letter, which can't be done if Feisul's enemies discover who carries it.
If Ramsden, Jeremy, Narayan Singh and I start for Damascus, pretending that one or other of us has the letter concealed on his person, and if a woman really carries it, we'll manage.

Is Mabel Ticknor going to be the woman?
That's the point." "Too dangerous, Jim! Too dangerous!" Ticknor put in nervously.
"Pardon me, old man.

The danger is for us four, who pretend we've got the thing." "There are lots of other women and I've only got one wife!" objected Ticknor.
"We're pressed for time," Grim answered.

"You see, Ticknor, old man, you're a Cornstalk and therefore an outsider--just a medico, who saws bones for a living, satisfied to keep your body out of the poorhouse, your soul out of hell, and your name out of the newspapers.

Your wife is presumably more so.