And now, in conclusion, I will show some of the ordinary optical
experiments with Hertz waves, using as source either one of two
devices: either a 5 in. sphere with sparks to ends of a diameter (Fig.
19), an arrangement which emits 7 in. waves but of so dead-beat a
character that it is wise to enclose it in a copper hat to prolong them
and send them out in the desired direction, or else a 2 in. hollow
cylinder with spark knobs at ends of an internal diameter (Fig. 12).
This last emits 3 in. waves of a very fairly persistent character, but
with nothing like the intensity of one of the outside radiators.
As receiver there is no need to use anything sensitive, so I employ a
glass tube full of coarse iron filings, put at the back of a copper hat
with its mouth turned well askew to the source, which is put outside
the door at a distance of some yards, so that only a little direct
radiation can reach the tube. Sometimes the tube is put lengthways
in the hat instead of crossways, which makes it less sensitive, and
has also the advantage of doing away with the polarising, or rather
analysing, power of a crossway tube.
The radiation from the sphere is still too strong, but it can be
stopped down by a diaphragm plate with holes in it of varying size
clamped on the sending box (right-hand side of Fig. 21).
_Reflection._
Having thus reduced the excursion of the spot of light to a foot or
so, a metal plate is held as reflector, and at once the spot travels
a couple of yards. A wet cloth reflects something, but a thin glass
plate, if dry, reflects next to nothing, being, as is well known, too
thin to give anything but “the black spot.” I have fancied that it
reflects something of the 3 in. waves.
With reference to the reflecting power of different substances, it may
be interesting to give the following numbers showing the motion of the
spot of light when 8 in. waves were reflected into the copper hat, the
angle of incidence being about 45 deg., by the following mirrors:--
Sheet of window glass 0 or at most 1 division.
Human body 7 divisions.
Drawing board 12 ”
Towel soaked with tap-water 12 ”
Tea-paper (lead?) 40 ”
Dutch metal paper 70 ”
Tinfoil 80 ”
Sheet copper 100 and up against stops.
_Refracting Prism and Lens._
A block of paraffin about a cubic foot in volume is cast into the shape
of a prism with angles 75 deg., 60 deg., and 45 deg. Using the large
angle, the rays are refracted into the receiving hat (Fig. 21), and
produce an effect much larger than when the prism is removed.
An ordinary 9 in. glass lens is next placed near the source, and
by means of the light of a taper it is focussed between source and
receiver. The lens is seen to increase the effect by concentrating the
electric radiation.
_Arago Disc; Grating; and Zone-plate._
The lens helps us to set correctly an 18 in. circular copper disc in
position for showing the bright diffraction spot. Removing the disc,
the effect is much the same as when it was present, in accordance with
the theory of Poisson. Add the lens and the effect is greater. With a
diffraction grating of copper strips 2 in. broad and 2 in. apart, I
have not yet succeeded in getting good results. It is difficult to get
sharp nodes and interference effects with these sensitive detectors
in a room. I expect to do better when I can try out of doors, away
from so many reflecting surfaces; indoors it is like trying delicate
optical experiments in a small whitewashed chamber well supplied
with looking-glasses; nor have I ever succeeded in getting clear
concentration with this zone-plate having Newton’s rings fixed to it
in tinfoil. The coherer, at any rate in a room, does not seem well
adapted to interference experiments; it is probably too sensitive, and
responds even at the nodes, unless they are made more perfect than is
easily practicable. But really there is nothing of much interest now
in diffraction effects, except the demonstration of the waves and the
measure of their length. There was immense interest in Hertz’s time,
because then the wave character of the radiation had to be proved; but
every possible kind of wave must give interference and diffraction
effects, and their theory is, so to say, worked out. More interest
attaches to polarisation, double refraction, and dispersion experiments.
[Illustration: FIG. 22.--Zone-plate of Tinfoil on Glass. Every circular
strip is of area equal to central space.]
_Polarising and Analysing Grids._
Polarisation experiments are easy enough. Radiation from a sphere, or
cylinder, or dumb-bell is already strongly polarised, and the tube acts
as a partial analyser, responding much more vigorously when its length
is parallel to the line of sparks than when they are crossed; but a
convenient extra polariser is a grid of wires something like what was
used by Hertz, only on a much smaller scale; say an 18 in. octagonal
frame of copper strip with a harp of parallel copper wires (_see_ Fig.
21, on floor). The spark-line of the radiator (Fig. 20) being set at 45
deg., a vertical grid placed over the receiver reduces the reflection
to about one-half, and a crossed grid over the source reduces it to
nearly nothing.
Rotating either grid a little rapidly increases the effect, which
becomes a maximum when they are parallel. The interposition of a third
grid, with its wires at 45 deg., between two crossed grids, restores
some of the obliterated effect.
Radiation reflected from a grid is strongly polarised, of course, in a
plane normal to that of the radiation which gets through it. They are
thus analogous in their effect to Nicols, or to a pile of plates.
The electric vibrations which get through these grids are at right
angles to the wires. Vibrations parallel to the wires are reflected or
absorbed.
_Reflecting Paraffin Surface; Direction of Vibrations in Polarised
Light._
To demonstrate that the so-called plane of polarisation of the
radiation transmitted by a grid is at right angles to the electric
vibration,[17] _i.e._, that when light is reflected from the boundary
of a transparent substance at the polarising angle the electric
vibrations of the reflected beam are perpendicular to the plane of
reflection, I use the same paraffin prism as before; but this time I
use its largest face as a reflector, and set it at something near the
polarising angle. When the line of wires of the grid over the mouth
of the emitter is parallel to the plane of incidence, in which case
the electric vibrations are perpendicular to the plane of incidence,
plenty of radiation is reflected by the paraffin face. Turning the grid
so that the electric vibrations are in the plane of incidence, we find
that the paraffin surface set at the proper angle is able to reflect
hardly anything. In other words, the vibrations contemplated by Fresnel
are the electric vibrations; those dealt with by McCullagh are the
magnetic ones.
Thus are some of the surmises of genius verified and made obvious to
the wayfaring man.