The beginning of light-signaling is hidden in the obscurity of the past. Of course, the most primitive light-signals were wood fires, but it is likely that man early utilized the mirror to reflect the sun's image and thus laid the foundation of the modern heliograph. The Book of Job, which is probably one of the oldest writings available, mentions molten mirrors. The Egyptians in the time of Moses used mirrors of polished brass. Euclid in the third century before the Christian era is said to have written a treatise in which he discussed the reflection of light by concave mirrors. John Peckham, Archbishop of Canterbury in the thirteenth century, described mirrors of polished steel and of glass backed with lead. Mirrors of glass coated with an alloy of tin and mercury were made by the Venetians in the sixteenth century. Huygens in the seventeenth century studied the laws of refraction and reflection and devised optical apparatus for various purposes. However, it was not until the eighteenth century that any noteworthy attempts were made to control artificial light for practical purposes. Dollond in 1757 was the first to make achromatic lenses by using combinations of different glasses. Lavoisier in 1774 made a lens about four feet in diameter by constructing a cell of two concave glasses and filling it with water and other liquids. It is said that he ignited wood and melted metals by concentrating the sun's image upon them by means of this lens. About that time Buffon made a built-up parabolic mirror by means of several hundred small plane mirrors set at the proper angles. With this he set fire to wood at a distance of more than two hundred feet by concentrating the sun's rays. He is said also to have made a lens from a solid piece of glass by grinding it in concentric steps similar to the designs worked out by Fresnel seventy years later. These are examples of the early work which laid the foundation for the highly perfected control of light of the present time.