A simple magnifying glass, focusing the sun's rays, can scorch a piece of wood or set a scrap of paper on fire. Solar radiation can also be concentrated on a much larger scale. It can burn a hole through thick steel plate, for example, or simulate the thermal shock of a nuclear blast. It can, that is, with the help of a super reflector of the sort that has been set up by French scientists high in the Pyrenees. The world's largest solar furnace is a complex of nearly 20,000 mirrors. It can concentrate enough sunlight to create temperatures in excess of 6000 degrees Fahrenheit.

The furnace's appearance is as spectacular as its power. Its glittering eight-story-high reflector towers over very old houses. Anchored against a concrete office and laboratory building, the huge reflector consists of nearly 9000 separate mirrors. For the furnace to operate, these small mirrors must be adjusted so that their light will meet exactly, at a focal point 59 feet in front of the giant reflector.

Q. Underline the sentence which proves that setting the giant reflector is a delicate operation.