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