In the living-room, I switched on the lights in time to see some one disappear through an open window.  I ran to the window and looked out.  There was a man, half doubled up, running around the side of the house and into a clump of bushes, then apparently lost.  I shot out of the window and called.

My only answer was an imprecation and return volley that shattered the glass above my head.  I ducked hastily and fell flat on the floor, for in the light streaming out, I must have been a good mark.

I was not the only one who heard the noise.  The shots quickly awakened Elaine and she leaped out of bed and put on her kimono.  Then she lighted the lights and ran down-stairs.

The intruder had disappeared by this time and I had got up and was peering out of the window as she came breathlessly into the living-room.

“What’s the matter, Walter?” she asked.

“Some one broke into the house after those plans,” I replied.  “He escaped, but I got his picture, I think, by this device of Kennedy’s.  Let’s go into a dark room and develop it.”

There was no use trying to follow the man further.  To Elaine’s inquiry of what I meant, I replied by merely going over to the spot where I had hidden the camera and disconnecting it.

We went up-stairs where I had rigged up an impromptu dark room for my amateur photographic work some days before.  Elaine watched me closely.  At last I found that I had developed something.  As I drew the film through the hypo tray and picked it up, I held it to the red light.

Elaine leaned over and looked at the film with me.  There was a picture of a masked man, his cap down, in a startled attitude, his hands clapped to his face, completely hiding what the mask and cap did not hide.

“Well, I’ll be blowed!” I cried in chagrin at the outcome of what I thought had been my cleverest coup.

A little exclamation of astonishment escaped Elaine.  I turned to her.  “What is it?” I asked.

“The ring!” she cried.

I looked again more closely.  On the little finger of the left hand was a peculiar ring.  Once seen, I think it was not readily forgotten.  “The ring!” she repeated excitedly.  “Don’t you remember—­that ring?  I saw it on Mr. Del Mar’s hand—­at his house--this afternoon!”

I could only stare.

At last we had a real clue!

In his bungalow, Del Mar at that moment threw down his hat and tore off his mask furiously.

What had he done?

For a long time he sat there, his chin on his hand, gazing fixedly before him, planning to protect himself and revenge.

CHAPTER XVI

THE DISAPPEARING HELMETS

It was early the following morning that, very excited, Elaine and I showed Aunt Josephine the photograph which we had snapped and developed by using Kennedy’s trick method.