The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million
John Stuart Mill · en
Accepting the theory of vibrations, and applying it to the
elucidation of the phenomena of light--it is unnecessary, we
think, to believe that a ray of _white_ light _contains_ rays
in _a state of colour_. It is said that if we divide a circular
surface into parts, and paint the various colours in the order
and proportions in which they occur in the refracted ray, and
then spin the circle with great velocity, the colours will blend
and appear _white_. But such is not the case; the result is in
some degree an illusion, arising out of the sudden removal of the
impression made upon the eye by the colours; and if a piece of
white paper be held by the side of the coloured circle in motion,
the latter will be found to be _grey_. When it is remembered
that in colouring a white surface with thin colours, the white
materially qualifies the colours, it must be admitted that the
experiment fails to support the assertion that the colours of
the spectrum produce white. But there can be no difficulty in
understanding that a ray of light undergoing _refraction_, becomes
divided into minor rays, which _differing in their degrees of
refrangibility_, vary also in the _velocity of their vibrations_,
and produce the several sensations of colour.
466. _Why is a substance white?_
Because it reflects the light that falls upon it _without altering its
vibrations._
467. _Why is a substance black?_
Because it _absorbs the light_ and _puts an end to the vibrations_.
468. _Why is the rose red?_
Because it imparts to the light that falls upon it that _change in its
vibratory condition_, which produces on our eyes the _sensation of
redness_.
469. _Why is the lily white?_
Because it reflects the light without altering its vibrations.
470. _Why is the primrose yellow?_
Because, though it receives white light, it alters its vibrations to
44,000 in an inch, and 535,000,000,000,000 in a second, and this is
the velocity of vibration which produces upon the eye a _sensation_ of
_yellow_.
[Verse: "But if thine eye be evil, thy whole body shall be full of
darkness. If therefore the light that is in thee be darkness, how great
is that darkness."--MATT. V.]
471. _Why are there so many varieties of colour and tint in the various
objects in nature?_