“Well,” Perenna continued, “in spite of every obstacle, the letters continued to come, as though they could not help it.  And thus the reason of their coming gradually dawned upon me:  they came mechanically, by some invisible process set going once and for all and working with the blind certainty of a physical law.  This was a case not of a conscious intelligence and will, but just of material necessity....  It was the clash of these two ideas—­the idea of the hatred pursuing the innocent and the idea of that machinery serving the schemes of the ’hater’—­it was their clash that gave birth to the little spark of light.  When brought into contact, the two ideas combined in my mind and suggested the recollection that Hippolyte Fauville was an engineer by profession!”

The others listened to him with a sort of uneasy oppression.  What was gradually being revealed of the tragedy, instead of relieving the anxiety, increased it until it became absolutely painful.

M. Desmalions objected: 

“Granting that the letters arrived on the dates named, you will nevertheless have noted that the hour varied on each occasion.

“That is to say, it varied according as we watched in the dark or not, and that is just the detail which supplied me with the key to the riddle.  If the letters—­and this was an indispensable precaution, which we are now able to understand—­were delivered only under cover of the darkness, it must be because a contrivance of some kind prevented them from appearing when the electric light was on, and because that contrivance was controlled by a switch inside the room.  There is no other explanation possible.

“We have to do with an automatic distributor that delivers the incriminating letters which it contains by clockwork, releasing them only between this hour and that on such and such a night fixed in advance and only at times when the electric light is off.  You have the apparatus before you.  No doubt the experts will admire its ingenuity and confirm my assertions.  But, given the fact that it was found in the ceiling of this room, given the fact that it contained letters written by M. Fauville, am I not entitled to say that it was constructed by M. Fauville, the electrical engineer?”

Once more the name of M. Fauville returned, like an obsession; and each time the name stood more clearly defined.  It was first M. Fauville; then M. Fauville, the engineer; then M. Fauville, the electrical engineer.  And thus the picture of the “hater,” as Don Luis said, appeared in its accurate outlines, giving those men, used though they were to the strangest criminal monstrosities, a thrill of terror.  The truth was now no longer prowling around them.  They were already fighting with it, as you fight with an adversary whom you do not see but who clutches you by the throat and brings you to the ground.

And the Prefect of Police, summing up all his impressions, said, in a strained voice: