Two channels feed the beam. One is where your hand is, left to right. The other is up and down. The screen plots one against the other and holds the light where you have been. Move slowly in a circle and you draw a clean ellipse — scribble, and you are noise.
Move your cursor across the screen
How far your hand sits from left to right becomes a voltage on the horizontal plates. Sweep left and the beam swings left; hold in the middle and it rests on the centre line. Nothing is recorded — the plate reads you continuously.
Up and down drives the vertical plates. With time thrown away and X plotted straight against Y, the dot no longer marches across the screen — it goes wherever you are, and the trail it leaves is the shape of your motion.
If your X and Y wobble at a simple whole-number ratio, the trace closes on itself and stops drifting — a standing figure. The panel names it and lights LOCK. Break the rhythm and it opens back into a wandering line.
Every closed figure is a ratio of frequencies and a phase. Nathaniel Bowditch drew these in 1815, decades before Lissajous put his bow to them; move your hand to their timing and the same shapes stand up on the screen above.
P31 · ~40 ms
The screen is coated in P31 phosphor. When the electron beam strikes it, the coating fluoresces a sharp yellow-green, then keeps glowing for a beat after the beam has moved on — persistence. That afterglow is why a fast figure hangs in the air as a whole shape instead of a single travelling dot.
Turn persistence up and the trail lingers, painting your path into a solid loop; turn it down and only the last instant survives, so you see the raw dot chasing your hand. Either way the light is always fading — the screen forgets you the moment you stop.
No footage, no recording. Every glow is computed in the moment from where your cursor is, sampled sixty times a second.
Each frame the beam eases toward your pointer and its position is pushed into two short buffers — one for X, one for Y — holding the last second and a half of your motion. The trail is real phosphor physics faked honestly: the whole screen is dimmed a hair every frame, and only the newest segment is drawn back in bright and additive, so old light decays away exactly as a coating would.
To name the figure, the bench counts how many times each channel crosses its own average — that gives a frequency for X and for Y, and their ratio reduces to the small whole numbers you see. The phase is the correlation between the two channels: perfectly out of step traces a circle, perfectly in step traces a diagonal line. A steadiness score weighs how evenly you are moving; move cleanly and it locks, scribble and it collapses to noise.