See the scan at google books.
||"New Pocket Cyclopædia: or, Elements of Useful
Knowledge, Methodically Arranged, with Lists of Select Books On every
important Subject of Learning and Science; designed for the Higher
Classes in Schools, and for Young Persons in General. By John Millard,
Assistant-Librarian of the Surry Institution. Second Edition, with many
important additions and corrections. London: Printed for Sherwood, Neely,
and Jones, Paternoster Row. 1813."
The illuminator is a piece of solid
glass, of a circular or elliptical form at the base; but the circular form
is the most productive of light, and the strongest against accident: it
is convex on the side to be presented outwards, to receive and condense
the rays of light, and has a flat or plane surface on the inside of the
room or apartment which it is intended to light. It is, or approaches to
a segment of a sphere, or spheroid; it is, in fact a lens; both sides may,
in general, be left polished; but when the illuminator is to be placed in
a situation where any danger may be apprehended of its being acted upon
as a burning glass, one side, at least, should be ground or roughed.
Its size is various according to the purpose, or situation, for which it
is designed, and its convexity is increased or diminished, according to
the size required. The ordinary dimensions are a base, of about five
inches in diameter, to one half-inch in height, from the centre of the
base; the illuminator is fixed in a square or circular frame, made of
wood or of metal, with glaziers' putty, or other cement.¹
¹The patent illuminators have been introduced, with
complete success, into private dwellings, instead of sky-lights; and
particularly, in the ceilings of cellars and under-ground offices.
Carriages have passed over these illuminators daily, without breaking
them, or producing any bad effect.