).

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
rece