Fresnel's Wonderful Concentrator

Fresnel's Wonderful Concentrator is a spell that concentrates things: e.g. the hate of hydrophobes in The Colour of Magic, parodying Roundworld's Fresnel lens, which concentrates light. This is then used to form a solid disc upon which it is safe to fly above the water, the speed and direction of the disc being controlled by the hydrophobes aboard.