We then use a new detector with four central squares. It is placed so that if the disk is at just the right height, and the incoming beam parallel, then the light striking the detector is precisely circular. This gives the same signal to each of the four squares
But if the disk moves downward, the beam pattern shifts inward, producing a horizontal ellipse of light on the detector. Then the two side squares get more light and they signal the electromagnet to lower the lens.
On the other hand, if the disk rises, the beam on the detector shits to a vertical ellipse. The stronger signal to the top and bottom squares then signal the lens to lift.
