might be supposed to perform a complete rotation. True, the
light-particle is smaller than the planet, and the distance _d_,
instead of being a million and a half of miles, is a little over the
ninety thousandth of an inch. But the two conceptions are, in point of
intellectual quality, identical.

Imagine, then, a particle entering the film of air where it possesses
this precise thickness. To enter the film, its attracted end must be
presented. Within the film it is able to turn _once_ completely round;
at the other side of the film its attracted pole will be again
presented; it will, therefore, enter the glass at the opposite side of
the film _and be lost to the eye_. All round the place of contact,
wherever the film possesses this precise thickness, the light will
equally disappear--we shall therefore have a ring of darkness.

And now observe how well this conception falls in with the law of
proportionality discovered by Newton. When the thickness of the film
is 2 _d_, the particle has time to perform, _two_ complete rotations
within the film; when the thickness is 3 _d, three_ complete
rotations; when 10 _d, ten_ complete rotations are performed. It is
manifest that in each of these cases, on arriving at the second
surface of the film, the attracted pole of the particle will be
presented. It will, therefore, be transmitted; and, because no light
is sent to the eye, we shall have a ring of darkness at each of these
places.

The bright rings follow immediately from the same conception. They
occur between the dark rings, the thicknesses to which they correspond
being also intermediate between those of the dark ones. Take the case
of the first bright ring. The thickness of the film is ½_d_; in this