Mathematical tracts on the lunar and planetary theories, the figure of the earth, precession and nutation, the calculus of variations, and the undulatory theory of optics.

RELATION OF COLOUR TO REFRANGIBILITY. 389
the former: and as these would enter the eye in such rapid
succession that we could not distinguish them, we should
only perceive the combined effect, which would be a uniform
white.
184. We have always spoken of the colour of a ray as
if it alone were sufficient to identify the nature of the ray.
Perhaps, however, it would have been better to consider a ray
as defined by its refrangibility. The remarkable experiment
of Fraunhofer (84), from which it appeared that the interruptions in the spectrum formed by interference correspond
exactly to those in the spectrum formed by refraction, seems
decisive as to this point, that rays of the same refrangibility
are produced by waves of the same length. It has, however,
long been the opinion of some philosophers that there are
rays of different colours which have the same degree of
refrangibility, and that there are rays of the same colour
with different degrees of refrangibility. Taking this as established, the conclusion seems to be, that colour does not
depend on the length of a wave, but probably on some other
circumstance, as perhaps the nature of the vibration. The
law of vibration may be, not that of a cycloidal pendulum
(as we have all along supposed), but something slightly
different. It may be that the effect of an absorbing medium
is to suppress all that part of the vibration which follows
that law, and to allow only the other to pass. These, however, are very vague conjectures, which can scarcely be examined till our knowledge of the subject in question is much
more extensive than it is at present.