We sat in silence, listening, horrified.

“There is still another matter,” Kennedy went on, after a moment.  “The fire in the negative vault this morning was incendiary.  I have proved to the satisfaction of several of us that a bomb was constructed of wet phosphorus and old film and placed in the vault by trickery four days ago, the same day Stella Lamar was killed.  Through a miscalculation the phosphorus was slow in drying and the fire did not occur until to-day.  Thanks to that fact I have in my possession a bit of negative which the murderer very likely wished to have destroyed; in fact, I believe its destruction to be the motive in planning the fire in the vault.”  He faced the operator.  “Ready to run the negative?”

“Yes, sir!”

Kennedy pressed the button and when the projection machine threw its picture upon the screen I saw something such as I had never imagined before.  Everything was black which should have been white and everything white which should have been black.  The two extremes shaded into each other in weird fashion.  In fact it was uncanny to watch a negative projected and I followed, fascinated.

“This is a film made with the co-operation of Doctor Nagoya of the Castleton Institute and I am told by Mr. Manton that it is one of the finest snake pictures ever made.”  Kennedy spoke fast, so that we would get the full benefit of his explanation and so that it would not be necessary to subject the negative to the wear and tear of the sprocket wheels in the projection machine again.  “I am running this for you to show you the action of the rattlesnake, whose venom was used to kill Miss Lamar, and to give you an idea of the source of the murderer’s knowledge of snake poison.”

At this moment Doctor Nagoya, whom I could barely recognize in the inverted photography, seized one of the rattlers.  It was a close-up and we could see the reptile dart out its forked tongue, seeking to get at the hands of the Japanese, locked firmly about its neck.  Then another man walked into the picture, holding a jar.  At once the snake struck at the glass.  As it did so it was possible to see drops of the venom projected into the jar.

Other details followed and there were views of other sorts and breeds of snakes, from the poisonous to the most harmless.  The principal scene, however, had been the one showing the venom.

“Lights up!”

The operator threw the switch again, stopping the film and at the same time lighting the projection room.  Kennedy stepped forward and turned to face us.

“There was this negative in the vaults.”  He spoke rapidly.  “It bore a certain name on the film, as editor.  Some one knew that proof of the possession of this knowledge of snakes might prove a powerful link in the chain against him.  If that had been a positive instead of a negative, you would have recognized Doctor Nagoya’s ‘assistant.’  There was a double motive in blowing that vault—­to destroy the company and to protect himself.  In fact, all the rest of the negative was destroyed.  Only by chance I saved this piece—­the very one that he wanted to destroy.”