his services, and needless to say, the boot-blacking and spittoon-cleaning ceased immediately. The man was too valuable to be allowed to spend his time in such duties.
Finally, the doctor, needing a confidential man to send to Africa, had entrusted Keene with the mission. Poor Pep Perkins was brokenhearted at his departure, because her admiration for this unique specimen of a man was unbounded. She had at last found a man who made her heart go "tick-tack!"
Keene was now back from Africa, and it was after his business report that he exclaimed:
"Doctor Hackensaw, I've got something peculiar to show you."
"What is it?" asked the Doctor.
"It's a little memento I brought back from my African trip. As you know, I had with me some of the special cameras you invented for taking photographs at night without the need of flash-lights."
"Yes," said Doctor Hackensaw. "I gave you photographic plates of two kinds. I gave you plates that were sensitive to electric, emanations so that you could take photos of the 'aura' that surrounds living beings."
"Precisely," said Silas. "What you call the Z-ray plates. Well, the curious thing I have to show you is one of the Z-ray photographs I took near Mongalla. I think you will find it rather curious!"
So saying, Phessenden Keene took from his pocket a photograph which he handed to the doctor, and Pep left cleaning her typewriter in order to get a better view.
A Young Girl and Her Aura
THE photograph represented what appeared to be a beautiful young girl in a state of nature, but surrounded by an aura of electric emanations.
"Well, what is there peculiar about this?" asked the doctor. "It's just an ordinary photograph of a young lady, taken on a special plate in order to show the 'aura'".
"No, indeed," replied Keene. "I took that snapshot in bright sunlight, and not a trace of a girl could I see. It was a bird I was photographing and I hadn't the faintest idea there was a girl anywhere near me. Doctor Hackensaw, do you believe there are such things as invisible creatures?"
"Well, yes there are. In the water there are certain animals like jelly-fishes that are so transparent that they are practically invisible. Among the animalcules too, there are many whose small size renders them invisible, and there are some that are so transparent that we cannot see them until they are stained even with a microscope. That is what makes it so difficult to discover the specific microbe that causes a disease. We must find some stain that will make the microbe visible, and this isn't always easy. The celebrated Ehrhardt had to try no less than 606 different stains before he found one that would color the microbes that cause syphilis. Once he found the proper stain, however, he was able to incorporate drugs with it, and was thus enabled to have his drugs carried into the bodies of the microbes. But, while invisible beings are common in the microscopic world, we know of no large invisible animals."
"Then the original of this photograph is the first one," said Keene, "and I wish to ask your permission to return to Africa and try to capture her."
"Why didn't you try while you were over there?"
"I unfortunately didn't develop the negative until after my return to the United States."
"Well," assented the Doctor. "A search for an invisible girl is worse than a search for a needle in a haystack, but the thing is so curious that we ought to make the attempt. I'll order my rapid aeroplane and Pep and I will go with you."
CHAPTER III
Bunches of Bananas for Bait
HERE we are, Doctor," exclaimed Phessenden Keene. "This is the very spot where I took the snapshot, as you can see by comparing it with the photograph."
"Even now," said Pep, "I don't see how we can ever hope to find the invisible girl. Pep spoke of its being as hard as looking for a needle in a haystack, but it seems to me much harder."
"Yes," assented the doctor, "but to a scientist there would he no difficulty in finding a needle in a haystack. He would merely spread out the hay and pass a strong electro-magnet over it, and in a few minutes the needle would be found clinging to the magnet."
"Perhaps so," said Pep, "but at least you can sen the needle when you do find it. Here we can't see the girl and even if she were right in front of us and we took a snapshot of her with the Z-rays, she would probably be gone by the time the negative was developed."
Doctor Hackensaw smiled. "You forget, Pep," said he, "that we have other senses besides the sense of sight. Besides there are ways of making the invisible visible. Don't imagine for an instant that I have come unprepared. I have in fact several strings to my how. You will remember that we stopped awhile in Mongalla and I heard news there that will be of use to us. Hunting parties out after lions or elephants have noticed the mysterious disappearance of their provisions. Bunches of bananas disappear, and also the strips of hippopotamus meat that they hang up to dry. This knowledge will be very useful to us. The girl evidently has no way of obtaining provisions except by stealing them, hence a bunch of fine bananas would make a tempting bait."
"Oh!" exclaimed Pep, "that's the reason you loaded up the aeroplane with such quantities of bunches of bananas!"
"Precisely. Our first job is to locate the girl. To do this we will hang up small bunches of bananas at likely spots near the White Nile, where she must go for water. And when tomorrow we find one or two of the bunches missing we day strongly suspect one invisible girl of being the culprit."
It was no easy task tramping through the wilds, for caution was necessary, as lions, leopards and elephants abounded in the region and even the crocodiles were dangerous, the post-boat captain having informed our adventurers that he had had two of his men devoured by crocodiles the past year.
