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 depends on the rectilineal
propagation of light. Make a small hole in a closed window-shutter,