When Huygens and Mariotte agree.

Edme Mariotte in the seventeenth century attributed halos to tiny ice prisms in the atmosphere. Christiaan Huygens attributed them to tiny spheres or cylinders. The two seemingly incompatible theories largely agree in their predictions for the common halos.

The legendary Rome halo displays.

The two Rome halo displays of 1629 and 1630 are prominent in the early halo literature, and the 1629 display is still cited today for having contained a 28 degrees circular halo. We have examined seventeenth century correspondence and publications in order to learn as much as possible about the existing documentation of the two displays. We find the documentation to be too weak to support a definitive interpretation of either display, and we see little evidence for a 28 degrees halo or for other rare halos.

Maximum deviation of light in a transparent wedge.

The maximum is found for the deviation of light passing through a transparent wedge of refractive index n and wedge angle alpha. The methods are conceptual and geometric, and they require very little calculation. There turn out to be two qualitatively different ray path candidates for maximum deviation, and the geometric approach leads naturally to a criterion involving n and alpha that decides between the two candidates.