Six Lectures on Light | Page 3

by means of it, to illustrate the growth of scientific knowledge
under the guidance of experiment. I wish, in the first place, to make
you acquainted with certain elementary phenomena; then to point out to
you how the theoretical principles by which phenomena are explained
take root in the human mind, and finally to apply these principles to the
whole body of knowledge covered by the lectures. The science of
optics lends itself particularly well to this mode of treatment, and on it,

therefore, I propose to draw for the materials of the present course. It
will be best to begin with the few simple facts regarding light which
were known to the ancients, and to pass from them, in historic
gradation, to the more abstruse discoveries of modern times.
All our notions of Nature, however exalted or however grotesque, have
their foundation in experience. The notion of personal volition in
Nature had this basis. In the fury and the serenity of natural phenomena
the savage saw the transcript of his own varying moods, and he
accordingly ascribed these phenomena to beings of like passions with
himself, but vastly transcending him in power. Thus the notion of
_causality_--the assumption that natural things did not come of
themselves, but had unseen antecedents--lay at the root of even the
savage's interpretation of Nature. Out of this bias of the human mind to
seek for the causes of phenomena all science has sprung.
We will not now go back to man's first intellectual gropings; much less
shall we enter upon the thorny discussion as to how the groping man
arose. We will take him at that stage of his development, when he
became possessed of the apparatus of thought and the power of using it.
For a time--and that historically a long one--he was limited to mere
observation, accepting what Nature offered, and confining intellectual
action to it alone. The apparent motions of sun and stars first drew
towards them the questionings of the intellect, and accordingly
astronomy was the first science developed. Slowly, and with difficulty,
the notion of natural forces took root in the human mind. Slowly, and
with difficulty, the science of mechanics had to grow out of this notion;
and slowly at last came the full application of mechanical principles to
the motions of the heavenly bodies. We trace the progress of astronomy
through Hipparchus and Ptolemy; and, after a long halt, through
Copernicus, Galileo, Tycho Brahe, and Kepler; while from the high
table-land of thought occupied by these men, Newton shoots upwards
like a peak, overlooking all others from his dominant elevation.
But other objects than the motions of the stars attracted the attention of
the ancient world. Light was a familiar phenomenon, and from the
earliest times we find men's minds busy with the attempt to render
some account of it. But without _experiment_, which belongs to a later
stage of scientific development, little progress could be here made. The
ancients, accordingly, were far less successful in dealing with light than

in dealing with solar and stellar motions. Still they did make some
progress. They satisfied themselves that light moved in straight lines;
they knew also that light was reflected from polished surfaces, and that
the angle of incidence was equal to the angle of reflection. These two
results of ancient scientific curiosity constitute the starting-point of our
present course of lectures.
But in the first place it will be useful to say a few words regarding the
source of light to be employed in our experiments. The rusting of iron
is, to all intents and purposes, the slow burning of iron. It develops heat,
and, if the heat be preserved, a high temperature may be thus attained.
The destruction of the first Atlantic cable was probably due to heat
developed in this way. Other metals are still more combustible than
iron. You may ignite strips of zinc in a candle flame, and cause them to
burn almost like strips of paper. But we must now expand our
definition of combustion, and include under this term, not only
combustion in air, but also combustion in liquids. Water, for example,
contains a store of oxygen, which may unite with, and consume, a
metal immersed in it; it is from this kind of combustion that we are to
derive the heat and light employed in our present course.
The generation of this light and of this heat merits a moment's attention.
Before you is an instrument--a small voltaic battery--in which zinc is
immersed in a suitable liquid. An attractive force is at this moment
exerted between the metal and the oxygen of the liquid; actual
combination, however, being in the first instance avoided. Uniting the
two ends of the battery by a thick wire, the attraction is satisfied, the
oxygen unites