“Just what do you mean?” asked Kennedy, gently.

“I don’t know.  Mother has been so different lately.  And now, every night, she goes out.”

“Where?” encouraged Kennedy, realizing that his plan was working.

“I don’t know.  If she would only come back looking happier.”  She was sobbing, convulsively, over she knew not what.

“Miss Blakeley,” said Kennedy, taking her hand between both of his, “only trust me.  If it is in my power I shall bring you all out of this uncertainty that haunts you.”

She could only murmur her thanks as we left.

“It is strange,” ruminated Kennedy, as we sped across the city again to the laboratory.  “We must watch Mrs. Blakeley.”

That was all that was said.  Although I had no inkling of what was back of it all, I felt quite satisfied at having recognized the mystery even on stumbling on it as I had.

In the laboratory, as soon as he could develop the skiagraphs he had taken, Kennedy began a minute study of them.  It was not long before he looked over at me with the expression I had come to recognize when he found something important.  I went over and looked at the radiograph which he was studying.  To me it was nothing but successive gradations of shadows.  But to one who had studied roentgenography as Kennedy had each minute gradation of light and shade had its meaning.

“You see,” pointed out Kennedy, tracing along one of the shadows with a fine-pointed pencil, and then along a corresponding position on another standard skiagraph which he already had, “there is a marked diminution in size of the sella turcica, as it is called.  Yet there is no evidence of a tumor.”  For several moments he pondered deeply over the photographs.  “And it is impossible to conceive of any mechanical pressure sufficient to cause such a change,” he added.

Unable to help him on the problem, whatever it might be, I watched him pacing up and down the laboratory.

“I shall have to take that picture over again—­under different circumstances,” he remarked, finally, pausing and looking at his watch.  “To-night we must follow this clue which Cynthia has given us.  Call a cab, Walter.”

We took a stand down the block from the Blakeley mansion, near a large apartment, where the presence of a cab would not attract attention.  If there is any job I despise it is shadowing.  One must keep his eyes riveted on a house, for, once let the attention relax and it is incredible how quickly any one may get out and disappear.

Our vigil was finally rewarded when we saw Mrs. Blakeley emerge and hurry down the street.  To follow her was easy, for she did not suspect that she was being watched, and went afoot.  On she walked, turning off the Drive and proceeding rapidly toward the region of cheap tenements.  She paused before one, and as our cab cruised leisurely past we saw her press a button, the last on the right-hand side, enter the door, and start up the stairs.