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
Light, according to Newton, is the effect of luminous particles which
dart from the surfaces of bodies in all directions. According to this
theory, the solar light which we receive would _depart from the sun and
travel to the earth_.
According to Huyghens, light is caused by an _infinitely elastic ether,
diffused through all space_. This ether, existing everywhere, is
_excited into waves, or vibrations, by the luminous body_.
The theory of light is so undetermined that neither the views
of Newton, nor those of Huyghens, can be said to be exclusively
adopted. Writers upon natural philosophy seize hold of either or
both of those theories, as they present themselves more or less
favourably in the explanation of natural phenomena. In "_The
Reason Why,"_ as we have to speak of the _effects_ of light rather
than of its _cause_, we shall avoid, as far as possible, the
doubtful points. But let no one be discouraged by the fact that
the theory of light, as, indeed, of all the imponderable agents,
is imperfectly understood. Rather let us rejoice that there are
vast fields of discovery yet to be explored; and that light, the
most glorious and inspiring element in nature, invites us from the
sun, the moon, and the stars, and from the face of every green
leaf and variegated flower, to search out the wonders of its
nature, and further to exemplify the goodness and wisdom of God.
[Verse: "And God said, Let there be light: and there was light."]
452. _What is the distance of the sun from the earth?_
Ninety five millions of miles.
453. _At what rate of velocity does light travel?_
At the rate of 192,000 miles in a _second_, through our _atmosphere_;
and 192,500 miles in a _second_ through a _vacuum_.
454. _How long does light take to travel from the sun to the earth?_
Eight minutes and thirteen seconds.
455. _What is the constitution of the sun?_
It is a spherical body, 1,384,472 times larger than the earth.
456. _From what does the luminosity of the sun arise?_
From a luminous atmosphere, or, as M. Arago named it, _photosphere_,
which completely surrounds the body of the sun, and which is probably
_burning with great intensity_.
457. _What are the minor sources of light?_
Light may be produced by _chemical action_, by _electricity_, and by
_phosphoresence_, in the latter of which various agencies unite.
458. _What is a ray of light?_
A _ray_ of light is the _smallest portion_ of light which we can
recognise.
459. _What is a medium?_
A _medium_ is a body which affords _a passage for the rays_ of light.
460. _What is a beam of light?_
A _beam_ of light is a _group of parallel rays_.
461. _What is a pencil of light?_
A _pencil_ of light is a body of rays which _come from or move towards
a point_.
[Verse: "And God saw the light, that it was good: and God divided
the light from the darkness."--GEN. I.]
462. _What is the radiant point?_