We are dealing with a clever man, Watson.

This matter cuts very deep, and though I have not finally made up my mind whether it is a benevolent or a malevolent agency which is in touch with us, I am conscious always of power and design.

When our friends left I at once followed them in the hopes of marking down their invisible attendant.

So wily was he that he had not trusted himself upon foot, but he had availed himself of a cab so that he could loiter behind or dash past them and so escape their notice.

His method had the additional advantage that if they were to take a cab he was all ready to follow them.