coatings in the manner indicated in Fig. 22, for example, there is
some difficulty in fitting the parts, but these difficulties would not
exist if a great many bulbs were manufactured; otherwise the energy
can be conveyed through the glass as well as through a wire, and with
these high frequencies the losses are very small. Such illuminating
devices will necessarily involve the use of very high potentials, and
this, in the eyes of practical men, might be an objectionable feature.
Yet, in reality, high potentials are not objectionable--certainly not
in the least as far as the safety of the devices is concerned.
There are two ways of rendering an electric appliance safe. One is to
use low potentials, the other is to determine the dimensions of the
apparatus so that it is safe no matter how high a potential is used.
Of the two the latter seems to me the better way, for then the safety
is absolute, unaffected by any possible combination of circumstances
which might render even a low-potential appliance dangerous to life
and property. But the practical conditions require not only the
judicious determination of the dimensions of the apparatus; they
likewise necessitate the employment of energy of the proper kind. It
is easy, for instance, to construct a transformer capable of giving,
when operated from an ordinary alternate current machine of low
tension, say 50,000 volts, which might be required to light a highly
exhausted phosphorescent tube, so that, in spite of the high
potential, it is perfectly safe, the shock from it producing no
inconvenience. Still, such a transformer would be expensive, and in
itself inefficient; and, besides, what energy was obtained from it
would not be economically used for the production of light. The
economy demands the employment of energy in the form of extremely
rapid vibrations. The problem of producing light has been likened to
that of maintaining a certain high-pitch note by means of a bell. It
should be said a _barely audible_ note; and even these words would not
express it, so wonderful is the sensitiveness of the eye. We may
deliver powerful blows at long intervals, waste a good deal of energy,
and still not get what we want; or we may keep up the note by
delivering frequent gentle taps, and get nearer to the object sought
by the expenditure of much less energy. In the production of light, as
far as the illuminating device is concerned, there can be only one
rule--that is, to use as high frequencies as can be obtained; but the
means for the production and conveyance of impulses of such character
impose, at present at least, great limitations. Once it is decided to
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