(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.

Sec. 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
risin