On the 14th January 1916, Dr. J. C. Bose delivered a public lecture, on
Light Visible and Invisible, at the third Indian Science Congress held
at Lucknow, before a crowded audience which included the
Lieutenant-Governor (Sir James Meston).
Dr. Bose, in course of his lecture, spoke of the imperfection of our
senses. Our ear, for example, fails to respond to all sounds. There are
many sounds to which we are deaf. This was because our ear was tuned to
answer to the narrow range of eleven octaves of sound vibrations. He
showed a remarkable experiment of an artificial ear which remained
irresponsive to various sounds, but when a particular note, to which it
was tuned, was sounded even at the distant end of the hall, this ear
picked it up and responded violently. As there were sounds audible and
inaudible, so there were lights visible and invisible. The imperfection
of our eye as a detector of ether vibrations was, however, far more
serious. The eye could detect ether vibrations lying within a single
octave--between 400 to 800 billion vibrations per second. Comparatively
slow vibrations of ether did not affect our eye and the disturbances
they give rise to well-known as electric waves. The electric waves,
predicted by Maxwell, were discovered by Hertz. These waves were about
three metres long. They were about ten million times larger than the
beams of visible light. Dr. Bose showed that the three short electric
waves have the same property as a beam of light, exhibiting reflections,
refraction, even total reflection, through a black crystal, double
refraction, polarisation, and rotation of the plane of polarisation. The
thinnest film of air was sufficient to produce total reflection of
visible light with its extremely short wave lengths. But with the new
electric waves which he produced, Dr. Bose showed that the critical
thickness of air space determined by the refracting power of the prison
and by the wave length of electric oscillations. Dr. Bose determined the
index of refraction of electric waves for different materials, and
eliminated a difficulty which presented itself in Maxwell's theory as to
the relation between the index of refraction of light and the
di-electric constant of insulators. He also measured the wave lengths of
various oscillations. The order to produce short electric oscillations,
to detect them and study their optical properties, he had to construct a
large number of instruments. It was a hard task to produce very short
electric waves which had enough energy to be detected, but Dr. Bose
overcame this difficulty by constructing radiators or oscillators of his
own type, which emitted the shortest waves with sufficient energy. As a
receiver he used a sensitive metallic coherer, which in itself led to
new and important discoveries. When electric waves fall on a loose
contact between two pieces of metals, the resistance of the contact
changes and a current passes through the contact indicating the
existence of electrical oscillations. Dr. Bose discovered the surprising
fact that with potassium metal the resistance of the contact increases
under the action of electric waves and that this contact exhibits an
automatic recovery. He found further that the change of the metallic
contact resistance when acted upon by electric waves, is a function of
the atomic weight. These phenomena led to a new theory of metallic
coherers. Before these discoveries it was assumed that the particles of
the two metallic pieces in contact are, as it were, fused together, so
that the resistance decreases. But the increasing resistance appearing
for some elements, led to the theory that the electric forces in the
waves produced a peculiar molecular action or a re-arrangement of the
molecules, which may either increase or decrease the contact resistance.
--_Pioneer_,--16-1-1916.