Lenses were employed in lighthouses at a very early period.  When they were first made they were used for burning instruments, by collecting the rays of the sun.  It was seen, however, that they would equally collect the rays of a lamp.  They have of late years been very greatly improved by a celebrated glass manufacturer.  Great indeed has been the improvement in lighthouses.  Once upon a time they were simply high towers, which had on their summits open fireplaces, in which either wood or coal fires were burned.  They were often unserviceable at the very time their services were most required.  During a heavy gale, for instance, when the wind was blowing towards the land, it drove the flames of an open fire away from the direction in which they were most wanted to be seen.  Sometimes, in fog or rain, the glare of the fire was visible by refraction in the atmosphere, although the fire itself could not be seen.  Such was the tower of the North Foreland.  This lighthouse existed in 1636, and merely had a large glass lantern fixed on the top of a timber erection, which, however, was burnt in 1683.  Towards the end of the same century a portion of the present structure was raised, having an iron grate on the summit.  It being found difficult to keep a proper flame in windy or rainy weather, about 1782 it was covered in with a roof and large sash windows, and a coal fire was kept alight by means of enormous bellows, which the attendants worked throughout the night.