Yardley Hildebrand had it in mind to build an adequate library at the "Hundred"; the villain had his cultivated tastes, and he wanted something which should be unique of its kind. Since my regular profession was that of an architect he naturally consulted me. I sketched out my ideas, and they met with his approval; he offered me the commission, and I accepted with alacrity. Then he sailed away, leaving me to carry out the plans--those in my sketchbook and some others that I had not taken the trouble to show him. Modern physicists are just now beginning to talk about the invisible heat and light rays composed of high frequency vibrations. But long before Crooks gave the X-ray to the world I had discovered and had succeeded in isolating what I choose to call the Sigma ray. Some fine day it will be rediscovered, and the lucky man will get a new lot of capital letters to tack onto his name; and perhaps a ribbon for his buttonhole, and a pension from his grateful government. I shall not care; the Sigma ray has repaid me a thousandfold for the trouble I took to establish its existence; as a lethal agent it stands without a peer, instantaneously destructive to all forms of organic life. Naturally, I do not propose to state the formula by means of which I was enabled to construct a filter capable of segregating my beloved Sigma ray _from ordinary sunlight_. Ah, that statement is illuminating, is it not! Suffice it to say that my filter looks like common glass. It may be moulded so as to resemble the familiar bullseye lens; and, if desirable, it can be colored. Now do you begin to appreciate the significance of the stained glass window on the right of the great fireplace in the library of "Hildebrand Hundred," the one depicting the Israelitish spies carrying their clusters of purple grapes? If you choose to make an interesting experiment, arrange for the erection of a staging or an extension ladder outside the "Spy" window, so as to bring your eye on a level with the third grape in the upper row of the largest bunch. You will find that the line of your view, through this particular bullseye, impinges upon the head of any person who may chance to be sitting in the swivel-chair before the big, teakwood desk. As the chair is immovably secured to the floor by steel bolts passing through its mushroom base, it is evident that the relationship of the chair and of that particular bullseye will remain fixed; at any rate, one would have to go to some trouble to disturb it.