One of the earliest notices of the application of lenses to lighthouses is that recorded by SMEATON in his Narrative of the Eddystone Lighthouse, where he mentions a London optician, who, in 1759, proposed grinding the glass of the lantern to a radius of seven feet six inches; but the description is too vague to admit of even a conjecture regarding the proposed arrangement of the apparatus. About the middle of the last century, however, lenses were actually tried in several lighthouses in the south of England, and in particular at the South Foreland in the year 1752; but their imperfect figure and the quantity of light absorbed by the glass, which was of impure quality and of considerable thickness, rendered their effect so much inferior to that of the parabolic reflectors then in use, that after trying some strange combinations of lenses and reflectors, the former were finally abandoned. Lenses were also tried at the lights of Portland, Hill of Howth, and Waterford, by Mr Thomas Rogers, a glass manufacturer in London; who possessed, it is said, the art of blowing mirrors of glass, “and by a new method silvered over the convex side without quicksilver.”[51] [51] Hutchinson’s Practical Seamanship, p. 200. See also the notice of the spherical mirrors made by Messrs François and Letourneau of Paris in a subsequent part of this volume. The object to be attained by the use of lenses in a Lighthouse is, of course, identical with that which is answered by employing reflectors; and both instruments effect the same end by different means, collecting the rays which diverge from a point called the _focus_, and projecting them forward in a beam, whose axis coincides with the produced axis of the instrument. We have already seen that, in the case of _reflection_, this result is produced by the light being _thrown back_ from a surface so formed as to make all the rays to proceed in one and the same required direction. In the case of _refraction_, on the other hand, the rays pass through the refracting medium, and are _bent_ or _refracted_ from their natural course into that which is desired. The celebrated BUFFON, to prevent the great absorption of light by the thickness of the material, which would necessarily result from giving to a lens of great dimensions a figure continuously spherical, proposed to grind out of a solid piece of glass, a lens in steps or concentric zones. This suggestion of BUFFON regarding the construction of large burning glasses, was first executed, with tolerable success, about the year 1780, by the Abbé ROCHON; but such are the difficulties attending the process of working a solid piece of glass into the necessary form, that it is believed the only other instrument ever constructed in this manner, is that which was made by Messrs COOKSON of Newcastle-upon-Tyne, for the Commissioners of Northern Lighthouses.