The white light of the sun is well known to be composed of several
coloured rays; or rather, according to the theory of undulations, when
the rate at which a ray vibrates is altered, a different sensation
is produced upon the optic nerve. The analytical examination of
this question shows that to produce a red colour the ray of light
must give 37,640 undulations in an inch, and 458,000,000,000,000 in
a second. Yellow light requires 44,000 undulations in an inch, and
535,000,000,000,000 in a second; whilst the effect of blue results from
51,110 undulations within an inch, and 622,000,000,000,000 of waves in
a second of time.--_Hunt’s Poetry of Science._