interval the rotating particle can perform only half a rotation. When,
therefore, it reaches the second surface of the film, its repelled
pole is presented; it is, therefore, driven back and reaches the eye.
At all distances round the centre corresponding to this thickness the
same effect is produced, and the consequence is a ring of brightness.
The other bright rings are similarly accounted for. At the second one,
where the thickness is 1½_d_, a rotation and a half is performed; at
the third, two rotations and a half; and at each of these places the
particles present their repelled poles to the lower surface of the
film. They are therefore sent back to the eye, and produce there the
impression of brightness. This analysis, though involving difficulties
when closely scrutinised, enables us to see how the theory of fits may
have grown into consistency in the mind of Newton.

It has been already stated that the Emission Theory assigned a greater
velocity to light in glass and water than in air or stellar space; and
that on this point it was at direct issue with the theory of
undulation, which makes the velocity in air or stellar space greater
than in glass or water. By an experiment proposed by Arago, and
executed with consummate skill by Foucault and Fizeau, this question
was brought to a crucial test, and decided in favour of the theory of
undulation.

In the present instance also the two theories are at variance. Newton
assumed that the action which produces the alternate bright and dark
rings took place at a _single surface_; that is, the second surface of
the film. The undulatory theory affirms that the rings are caused by
the interference of waves reflected from both surfaces. This also has
been demonstrated by experiment. By a proper arrangement, as we shall
afterwards learn, we may abolish reflection from one of the surfaces