Instrument for solving Alhazen’s problem

The problem of Alhazen, a Persian mathematician from the 11th century, consisted in finding the point on the surface of a convex mirror at which a light ray from a given point must strike in order to be reflected to another given point. Even though he did not succeed in finding a geometric solution to the problem, Leonardo did design this ingenious instrument that could be used for determining a light ray’s point of incidence on a convex mirror, given the positions of the mirror’s center, of the light source, and of an observing eye.

On the flat surface, we can see a sectional portion of a spherical mirror, a small black cylinder representing the eye of the observer, and a similar white cylinder, indicating the light source. With the end of the short rod positioned at the eye, the movable slotted stick is adjusted over the light source until it intersects with the fixed slotted stick passing through the center of the mirror, at a point on the circumference of the mirror. This point of intersection found corresponds to the point of reflection that was being sought.