s work that did not
immediately result either in the production of practical apparatus, or
in valuable applications in the arts and sciences, or those which have
not even yet proved fruitful. Some discoveries and devices are so far
ahead of the times in which they are produced that several lifetimes
often pass before the world is ready to utilize them. Like immature or
unripe fruit, they are apt to die an untimely death, and it sometimes
curiously happens that, several generations after their birth, a
subsequent inventor or discoverer, in honest ignorance of their prior
existence, offers them to the world as absolutely new. The times being
ripe, they pass into immediate and extended public use, so that the
later inventor is given all the credit of an original discovery, and the
true first and original inventor remains unrecognized.

We will first examine Faraday's discovery of the relations existing
between light and magnetism. Though the discovery has not as yet borne
fruit in any direct practical application, yet it has proved of immense
value from a theoretical standpoint. In this investigation Faraday
proved that light-vibrations are rotated by the action of a magnetic
field. He employed the light of an ordinary Argand lamp, and polarized
it by reflection from a glass surface. He caused this polarized light to
pass through a plate of heavy glass made from a boro-silicate of lead.
Under ordinary circumstances this substance exerted no unusual action on
light, but when it was placed between the poles of a powerful
electro-magnet, and the light was passed through it in the same
direction as the magnetic flux, the plane of polarization of the light
was rotated in a certain direction.

Faraday discovered that other solid substances besides glass exert a
similar action on a beam of polarized light. Even opaque solids like
iron possess this property. Kerr has proved that a beam of light passed
through an extremely thin plate of highly magnetic iron has its plane of
polarization slightly rotated. Faraday showed that the power of rotating
a beam of polarized light is also possessed by some liquids. But what is
most interesting, in both solids and liquids, is that the direction of
the rotation of the light depends on the direction in which the
magnetism is passing, and can, therefore, be changed by changing the
polarity of the electro-magnet.

Faraday did not seem to thoroughly understand this phenomenon. He spoke
as if he