386 LIEUT. DRUMMOND ON THE ILLUMINATION OF LIGHT-HOUSES. 
This plan was fortunately never very extensively adopted ; and in those light- 
houses belonging to the Trinity House, where it was tried, it has subsequently 
been discontinued, and the lenses replaced by reflectors. The North Foreland, 
however, under the management of the governors of Greenwich Hospital, still 
remains a solitary example of a method which cannot be too soon abandoned, 
more especially since the remedy is so easy, — merely to remove the lenses, 
and leave a free and unobstructed passage to the light of the reflectors. 
Another mode, differing from any of those now described, has lately been 
introduced into France by MM. Arago and Fresnel, which rivals the most 
powerful of our lights in brilliancy, and surpasses them in economy and facility 
of management. A large Argand lamp with four concentric wicks, the exte- 
rior of which is 3^ inches in diameter, occupies the centre of the light-house. 
Around this powerful light eight magnificent lenses 30 inches square are dis- 
posed, touching each other at the edges, and forming a hollow octagonal prism 
about the lamp. Above these, smaller lenses of a similar construction, but in 
the form of trapezoids, are placed, inclining towards the centre till their axes 
form angles of about 50° with the horizon, at which inclination their sides 
come into contact, and thus completely inclose the central light. By the inter- 
vention of plane mirrors, the beams of light issuing from the secondary lenses 
are rendered parallel to those of the principal ; but by the same means a hori- 
zontal deviation of about 7° is given to them, so that this addition to the light 
is made to contribute to the divergence and consequent duration of light when 
revolving, rather than to add to its brilliancy. The lens, which is plano-convex, 
is of a peculiar construction, being formed of separate rings or zones, whose 
convex surfaces preserve nearly the same curvature as if they constituted 
portions of one complete lens, the interior and useless part of the glass being 
removed ; so that a section of these zones resembles a wedge placed with 
the edge uppermost; one side, that next the lamp, being a straight line, the 
other an arc of a circle*. 
The idea of such a lens appears first to have occurred to the celebrated 
Biffon, when engaged in some experiments on burning-glasses ; but he sup- 
posed, what is not possible, that it might be ground out of one large piece of 
glass. Dr. Brewster, in an article on the same subject in the Edinburgh 
• I ri.s\ f.l, Mcmoire sur un nouveau systerae d'eclairage, lu a l’Academie des Sciences, 1822 . 
