the body to be heated; from there, and from nowhere else, it should be
radiated. It is, of course, out of the question to reach this
theoretical result, but it is possible by a proper construction of the
illuminating device to approximate it more or less.

For many reasons, the refractory body is placed in the centre of the
bulb, and it is usually supported on a glass stem containing the
leading-in wire. As the potential of this wire is alternated, the
rarefied gas surrounding the stem is acted upon inductively, and the
glass stem is violently bombarded and heated. In this manner by far
the greater portion of the energy supplied to the bulb--especially
when exceedingly high frequencies are used--may be lost for the
purpose contemplated. To obviate this loss, or at least to reduce it
to a minimum, I usually screen the rarefied gas surrounding the stem
from the inductive action of the leading-in wire by providing the stem
with a tube or coating of conducting material. It seems beyond doubt
that the best among metals to employ for this purpose is aluminium, on
account of its many remarkable properties. Its only fault is that it
is easily fusible, and, therefore, its distance from the incandescing
body should be properly estimated. Usually, a thin tube, of a diameter
somewhat smaller than that of the glass stem, is made of the finest
aluminium sheet, and slipped on the stem. The tube is conveniently
prepared by wrapping around a rod fastened in a lathe a piece of
aluminium sheet of the proper size, grasping the sheet firmly with
clean chamois leather or blotting paper, and spinning the rod very
fast. The sheet is wound tightly around the rod, and a highly polished
tube of one or three layers of the sheet is obtained. When slipped on
the stem, the pressure is generally sufficient to prevent it from
slipping off, but, for safety, the lower edge of the sheet may be
turned inside. The upper inside corner of the sheet--that is, the one