esultant spectrum may be regarded as an indefinite
number of images of the zone placed side by side. In the image before
dispersion we have _iris-rings_, the extinction of the light being
nowhere complete; but when the different colours are separated by
dispersion, each colour is crossed transversely by its own system of
dark interference bands, which become gradually closer with the
increasing refrangibility of the light. The complete spectrum,
therefore, appears furrowed by a system of continuous dark bands,
crossing the colours transversely, and approaching each other as they
pass from red to blue.

In the case of the plate of selenite, a slit is placed in front of the
polarizer, and the film of selenite is held close to the slit, so that
the light passes through the central zone of the film. As in the case
of Newton's rings, the image of the zone is crossed by iris-coloured
bands; but when subjected to prismatic dispersion, the light of the
zone yields a spectrum furrowed by bands of complete darkness exactly
as in the case of Newton's rings and for a similar reason. This is the
beautiful effect described by Mr. Spottiswoode as the fanlike
arrangement of the bands--the fan opening out at the red end of the
spectrum.

* * * * *

_MEASUREMENT OF THE WAVES OF LIGHT._

The diffraction fringes described in Lecture II., instead of being
formed on the retina, may be formed on a screen, or upon ground glass,
when they can be looked at through a magnifying lens from behind, or
they can be observed in the air when the ground glass is removed.
Instead of permitting them to form on the retina, we will suppose them
formed on a screen. This places us in a condition to understand, even
without trigonometry, the solution of the important problem of
measuring _the length_ of a wave of light.

We will suppose the screen so distant that the rays falling upon it
from the two margins of the slit are sensibly parallel. We have
learned in Lecture II. that the first of the dark bands corresponds to
a difference of marginal path of one undulation; the second dark band
to a difference of path of two undulations; the third dark band to a
difference of three undulations, and so on. Now the angular distance
of the bands from the centre is capable of exact measurement; this
distance depending, as already stated, on the width of the slit. With
a slit 1.35 millimeter wide,[29] Schwerd found the angular distance