It will be remembered that Mr. Palmer and his son met Mrs. Montague for the first time at the reception given by Mrs. Merrill; also that their attention was attracted to a lady who wore a profusion of unusually fine diamonds--a Mrs. Vanderheck. We know how Ray was introduced to her, and repeated her name as Vander_beck_, with an emphasis on the beck; how she started, changed color, and glanced at him curiously as he did so, and seemed strangely ill at ease while conversing with him afterward, and a little later abruptly took her leave. The next day, the young man communicated these suspicious circumstances to the private detective whom his father had employed to look up his stolen diamonds, and from that time Mrs. Vanderheck had been under close surveillance by that shrewd official. Mr. Rider was a very energetic man, and, by dint of adroit inquiries and observations, learned that she was a woman who devoted most of her time to social life--was very gay, very fond of dress and excitement of every kind. She did not, however, resemble in any way the Mrs. Vanderbeck who had conducted the robbery of the Palmer diamonds, although, he argued, she might easily enough be an accomplice. The detective interviewed Doctor Wesselhoff, who was now as eager as any one to assist in the discovery of those who so imposed upon him, and obtained a minute description of the other woman who had arranged for Ray Palmer to become an inmate of his institution, and he thought that possibly by the aid of a clever disguise, Mrs. Vanderheck might have figured as Mrs. Walton, the pretended mother of the pretended monomaniac. Consequently, energetic Mr. Rider had followed close upon her track, bound to discover her real character. She resided in a fine brown-stone mansion up town; sported an elegant carriage and a spanking pair of bays, and, to all appearances, possessed an unlimited bank account. She was sometimes attended on her drives by a gentleman, many years older than herself, who appeared to be something of an invalid, and who, as far as the detective could learn, was engaged in no business whatever. These latter facts increased his suspicions, for the reason that the woman who had robbed Mr. Palmer had wished to submit her selections to the sanction of an invalid husband. Disguised as a spruce young coachman, Mr. Rider managed to ingratiate himself with a pretty, but rather vain young servant-girl in Mrs. Vanderheck's employ, and by means of well-turned compliments, re-enforced now and then with some pretty gift, he managed to keep himself well posted regarding the mistress' movements and her social engagements, and then he diligently followed up his espionage by frequenting the many receptions and balls which she attended. At these places she was always magnificently attired and seldom wore any ornaments except diamonds, of which she appeared to possess an endless store, and all were of great beauty and value.