s in the past received
greater attention from philosophers and scientists than that involved in
the question as to "What is Light?" Indeed, it may truthfully be said,
that even to-day its exact character is not positively known. That it is
due like heat to some periodic wave motion in the Aether is known, but
the exact character of that wave motion has yet to be determined. As in
the case of heat, so in the case of light, there have been two theories
which have contended with each other for supremacy in endeavouring to
answer the question as to "What is Light?" Those two theories are known
as the Emission or Corpuscular Theory, and the Undulatory or Wave
Theory. The corpuscular theory was introduced and developed by Newton in
his work on _Optics_, which ranks second only to the _Principia_ as a
work revealing masterly research and scientific genius. Newton supposed
that a luminous or lighted body actually emitted minute particles, which
were shot out from the body with the velocity of light, that is, at the
rate of 186,000 miles per second. These minute particles he termed
corpuscles. In the work just referred to regarding this matter, he asks
the question, "Are not rays of light very small bodies emitted from
shining substances?" These small particles or corpuscles were supposed
by him to actually strike the retina of the eye, and so produce the
sensation of Sight, in the same way that odorous particles entering the
nostril, come into contact with the olfactory nerves and produce the
sensation of Smell. In order, however, to account for certain phenomena
of light, he was compelled to postulate an aetherial medium to fill all
space, in which his luminous corpuscles travelled, and which would
excite waves in that medium. In his eighteenth query on this point he
asks: "Is not the heat of a warm room conveyed through the vacuum by the
vibration of a much subtler medium than air, and is not this medium the
same with that medium by which light is reflected or refracted, and by
whose vibrations light communicates Heat to bodies, and is put into fits
of easy reflection and easy transmission?" The corpuscular theory,
however, received its death-blow when, in competition with the wave
theory of light, as developed by Young, it was found that the latter
theory satisfactorily accounted for certain phenomena as the refraction
of light, which the corpuscular theory did not adequately account for.
Even while Newton was developing his t