Six Lectures on Light | Page 4

with the metal, zinc is consumed, and heat, as usual, is
the result of the combustion. A power which, for want of a better name,
we call an electric current, passes at the same time through the wire.
Cutting the thick wire in two, let the severed ends be united by a thin
one. It glows with a white heat. Whence comes that heat? The question
is well worthy of an answer. Suppose in the first instance, when the
thick wire is employed, that we permit the action to continue until 100
grains of zinc are consumed, the amount of heat generated in the
battery would be capable of accurate numerical expression. Let the
action then continue, with the thin wire glowing, until 100 grains of
zinc are consumed. Will the amount of heat generated in the battery be
the same as before? No; it will be less by the precise amount generated

in the thin wire outside the battery. In fact, by adding the internal heat
to the external, we obtain for the combustion of 100 grains of zinc a
total which never varies. We have here a beautiful example of that law
of constancy as regards natural energies, the establishment of which is
the greatest achievement of modern science. By this arrangement, then,
we are able to burn our zinc at one place, and to exhibit the effects of
its combustion at another. In New York, for example, we may have our
grate and fuel; but the heat and light of our fire may be made to appear
at San Francisco.
[Illustration: Fig. 1.]
Removing the thin wire and attaching to the severed ends of the thick
one two rods of coke we obtain, on bringing the rods together (as in fig.
1), a small star of light. Now, the light to be employed in our lectures is
a simple exaggeration of this star. Instead of being produced by ten
cells, it is produced by fifty. Placed in a suitable camera, provided with
a suitable lens, this powerful source will give us all the light necessary
for our experiments.
And here, in passing, I am reminded of the common delusion that the
works of Nature, the human eye included, are theoretically perfect. The
eye has grown for ages towards perfection; but ages of perfecting may
be still before it. Looking at the dazzling light from our large battery, I
see a luminous globe, but entirely fail to see the shape of the
coke-points whence the light issues. The cause may be thus made clear:
On the screen before you is projected an image of the carbon points, the
whole of the glass lens in front of the camera being employed to form
the image. It is not sharp, but surrounded by a halo which nearly
obliterates the carbons. This arises from an imperfection of the glass
lens, called its _spherical aberration_, which is due to the fact that the
circumferential and central rays have not the same focus. The human
eye labours under a similar defect, and from this, and other causes, it
arises that when the naked light from fifty cells is looked at the blur of
light upon the retina is sufficient to destroy the definition of the retinal
image of the carbons. A long list of indictments might indeed be
brought against the eye--its opacity, its want of symmetry, its lack of
achromatism, its partial blindness. All these taken together caused
Helmholt to say that, if any optician sent him an instrument so
defective, he would be justified in sending it back with the severest

censure. But the eye is not to be judged from the standpoint of theory.
It is not perfect, but is on its way to perfection. As a practical
instrument, and taking the adjustments by which its defects are
neutralized into account, it must ever remain a marvel to the reflecting
mind.
§ 3. _Rectilineal Propagation of Light. Elementary Experiments. Law
of Reflection._
The ancients were aware of the rectilineal propagation of light. They
knew that an opaque body, placed between the eye and a point of light,
intercepted the light of the point. Possibly the terms 'ray' and 'beam'
may have been suggested by those straight spokes of light which, in
certain states of the atmosphere, dart from the sun at his rising and his
setting. The rectilineal propagation of light may be illustrated by
permitting the solar light to enter, through a small aperture in a
window-shutter, a dark room in which a little smoke has been diffused.
In pure air you cannot see the beam, but in smoky air you can, because
the light, which passes unseen through the air, is scattered and revealed
by the smoke particles, among which the beam pursues a straight
course.
The following instructive experiment