Remarkable technical application of illusion was the so-called "Spiral disk"used in the first television sets. Figure 1 you see a solid circle, the edges of which are scattered a dozen holes with clearance of 2 mm; the holes are evenly spaced along a spiral line, each the size of a clearance closer to the center than adjacent. This disc does not promises as if nothing special.
But install it on an axis, arrange the window in front of him and behind put the same size image (Fig. 2).
If now to lead the drive in rapid rotation, there will be an unexpected phenomenon: picture cover still drive becoming if it is rotating clearly visible in the front window. Slow the rotation of the image becomes vague and, finally, when you stop the disc disappears completely; now from the picture remains visible only what can be seen through the tiny two-hole.
Understand the secret of the mysterious effect of this disc. We will rotate the drive slowly and watch for sequentially passing each of the holes past the window. The outermost hole passes near the upper edge of the window; if the movement is fast, it will make visible the whole strip pictures, adjacent to its upper edge. The next hole, below the first, with the rapid passage of the field window will open a second strip of images related to the first (Fig. 3); the third hole will make visible the third strip, etc., At a sufficiently rapid rotation of the disc becomes, because of this, all visible picture; against the window as if cut out of the disk corresponding hole.
The Spiral disk is easy to manufacture himself; for his quick rotation can be used wound on its axis lace - it is better to use a small electric motor.