He took down a bottle filled with a dark blue liquid. This he held up to the light that poured in from the window.

"Here," said he, "is the bloodhound upon whom I depend to find and mark the parasite. It bears the rather formidable name in its present state of aqueous methylene-blue, and is in a two per cent. solution. Combined with it is a five per cent. solution of borax. I had a druggist send it in this morning."

This mixture he poured into the small vessel until the bottom was barely covered; then he added water until there was a layer of perhaps one centimeter in thickness, and the blue began to become transparent.

The alcohol had dried off the bit of gla.s.s by this time; and Ashton-Kirk took the fragment up with a pair of forceps and dipped it several times into the methylene stain; after this he pa.s.sed it through clear water until the blue paled to a greenish tinge. Then he took up a white disc of filter paper; placing this upon a stand he laid the gla.s.s upon it and carefully dried both sides, much as one would blot ink from a letter sheet.

"This process is what is called staining," said Ashton-Kirk, "and the method I have used is one recommended by Koch; it is somewhat similar to the older one of Mannaberg, but more rapid in result."

Out of a tube he dropped a single gem-like globule of cedar oil upon the blood smear; then he covered it with a small square of gla.s.s; upon this in its turn fell a second drop of the oil.

The whole was then placed in position under a microscope and fastened.

Then the secret agent brought out the lens. It glittered like a tiny diamond in a huge setting, and Fuller gazed at it fascinated.

"How you can see anything through a gla.s.s as small as that I can't understand," said he. "It looks like the point of an awl."

"It is a one-twelfth objective," replied the other, as he screwed the lens firmly down upon the cover gla.s.s, and thus embedded it, so to speak, in the globule of cedar oil.

"It is necessary," said he, "that the specimen be observed through the oil because the lens must be brought down directly upon the gla.s.s; without the oil the gla.s.s would be scratched and the whole thing ruined."

Then he set himself to the close study of what the tiny lens made plain; in a few moments he lifted his head with an exclamation of triumph.

"I have it!" he cried.

"What have you found?" asked Fuller eagerly.

"Evidence," answered Ashton-Kirk, triumphantly, "that will enable me to lay my hand upon the person who searched the library and clothing of Dr.

Morse."

"The murderer?"

"Perhaps he is that also--who knows?"

"But," demanded Fuller, "I don't quite understand."

Ashton-Kirk waved his hand toward the microscope, and Fuller applied his eye to it.

"What do you see?" asked the secret agent.

"A pale green circle," answered the other, "and it is crowded with irregularly shaped spots."

"Compare the circle with the dial of a watch and look closely at the point where the six should be."

"Yes," said Fuller.

"What do you see--at a very little distance from the edge?"

"There are some small blue spots; some are dark, the others lighter and more intense."

"That last is my proof," said Ashton-Kirk. Then as Fuller turned upon him a still inquiring look, he added:

"The indications have been that some member of Dr. Morse's household had a hand in his death. The house was secure at all points; it was not possible for any one to gain an entrance after the locking up. You might say: Suppose the criminal had entered the house before the time for locking up and remained concealed until he saw his opportunity? To that I would answer that we would have detected his method of departure. He should have left something unfastened behind him unless he had a confederate in the house. That the doors and windows, in every instance, were fast proves that this must be the case."

Fuller nodded his head.

"That's so," said he.

"Now let us take the members of the household one at a time. Miss Corbin----"

Fuller waved his hand.

"Oh, she's out of it," said he.

"Very well," said Ashton-Kirk, his white teeth showing in a smile. "Then let us take up Nanon. Here we have a severely religious woman--one who evidently detested her employer, but who served him well and had been many years in the family."

"It looks as though we'd have to pa.s.s her, too," said Fuller. "There is no reason why _she_ should murder Dr. Morse that I can see."

Again the other smiled.

"In this you agree with the newspapers, at any rate," said he. "None of them have found occasion to a.s.sociate her with the matter, either."

"I also agree with the papers in the matter of Warwick," said Fuller. "I know that it's best to start without preconceived notions, but I can't help thinking that, if he's not exactly the man, he knows quite a bit about it all."

"That he has unaccountably disappeared is a bad point against him,"

admitted Ashton-Kirk. "And that some one resembling him was seen stealing away in the night, carrying a hand-bag, is another and most damaging one. However, as you say, it is best not to start with preconceived notions; and until we are sure that the unknown _was_ Warwick, and that the bag he carried _was_ the missing bag, we'd better not accuse him."

There was a pause; the secret agent looked at the stained blood smear for a moment and then continued:

"There is still another person--the fourth and last. This person possessed the marked symptoms of a common complaint--chills followed by fever. To this person I know Dr. Morse gave quinine."

"Well?" asked Fuller, eagerly.

"Chills and fever are indications of malaria--quinine is the invariable remedy for that complaint. And the light blue spots which you see in that smear of blood," pointing to the microscope, "are the germs of that same disease."

For a moment Fuller stood as though transfixed.

"You have the man!" he cried at last. "You have him beyond the shadow of a doubt! To think," in great admiration, "that he should be found out in such an unusual way. Why, it is one of the----" Here he paused, the enthusiasm died from his face, and he added slowly: "But suppose that blood clot was not left upon the drawer pull at the time you think. The man may have been in the library during the afternoon upon a perfectly legitimate errand."

But Ashton-Kirk shook his head.

"No," said he. "It happened last night about the time of the murder. If it had been earlier the blood would have been dry and hard to the core."

"I see," said Fuller. "I recall that you were surprised at its having retained any softness, even at that. But there is something else. If Miss Corbin is sure that Drevenoff did not descend from the third floor, after once going to his room, how do you account for his presence in the library at that time?"

"Miss Corbin was in position to see Drevenoff as he ascended the back stairs. She did not see him descend, and so concluded that he could not have done so. As a matter of fact he could have gained the first floor without any trouble by pa.s.sing through some unoccupied rooms upon the third floor, and using the front or main staircase."

"Then that's it," declared Fuller. "He came down that way while the old servant was in the kitchen seeing to the coffee, did his work and went back to his room by the same route. But," with a puzzled look upon his face, "what in the world ever drew your attention to Drevenoff in the first place--that is, what made you think it might be his blood upon the handle of the drawer?"

"Do you recall that while I was examining the desk I stopped to listen?"