_Of Light._
Another most important agent in nature, and one that has a near
connexion with heat, is _light_, being emitted by all bodies in a state
of ignition, and especially by the sun, the great source of light and of
heat to this habitable world.
Whether light consists of particles of matter (which is most probable)
or be the undulation of a peculiar fluid, filling all space, it is
emitted from all luminous bodies in right lines.
Falling upon other bodies, part of the light is _reflected_ at an angle
equal to that of its incidence, though not by impinging on the
reflecting surface, but by a power acting at a small distance from it.
But another part of the light enters the body, and is _refracted_ or
bent _towards_, or _from_, the perpendicular to the surface of the new
medium, if the incidence be oblique to it. In general, rays of light
falling obliquely on any medium are bent as if they were attracted by
it, when it has a greater density, or contains more of the inflammable
principle, than the medium through which it was transmitted to it. More
of the rays are reflected when they fall upon a body with a small degree
of obliquity to its surface, and more of them are transmitted, or enter
the body, when their incidence is nearer to a perpendicular.
The velocity with which light is emitted or reflected is the same, and
so great that it passes from the sun to the earth in about eight
minutes and twelve seconds.
Rays of light emitted or reflected from a body entering the pupil of the
eye, are so refracted by the humours of it, as to be united at the
surface of the retina, and so make images of the objects, by means of
which they are visible to us; and the magnifying power of telescopes or
microscopes depends upon contriving, by means of reflections or
refractions, that pencils of rays issuing from every point of any object
shall first diverge, and then converge, as they would have done from a
much larger object, or from one placed much nearer to the eye.
When a beam of light is bent out of its course by refraction, all the
rays of which it consists are not equally refracted, but some of them
more and others less; and the colour which they are disposed to exhibit
is connected invariably with the degree of their refrangibility; the
red-coloured rays being the least, and the violet the most refrangible,
and the rest being more or less so in proportion to their nearness to
these, which are the extremes, in the following order, violet, indigo,
blue, green, yellow, orange, red.
These colours, when separated as much possible, are still contiguous;
and all the shades of each colour have likewise their separate and
invariable degrees of refrangibility. When separated as distinctly as
possible, they divide the whole space between them exactly as a musical
chord is divided in order to found the several notes and half notes of
an octave.
These differently-coloured rays of light are also separated in passing
through the transparent medium of air and water, in consequence of which
the sky appears blue and the sea green, these rays being returned, while
the red ones proceed to a greater distance. By this means also objects
at the bottom of the sea appear to divers red, and so do all objects
enlightened by an evening sun.
The mixture of all the differently-coloured rays, in the proportions in
which they cover the coloured image above mentioned, makes a _white_,
and the absence of all light is _blackness_.
By means of the different refrangibility of light, the colours of the
rainbow may be explained.
The distance to which the differently-coloured rays are separated from
each other is not in proportion to the mean refractive power of the
medium, but depends upon the peculiar constitution of the substance by
which they are refracted. The _dispersing power_ of glass, into the
composition of which _lead_ enters, is great in proportion to the mean
refraction; and it is proportionally little in that glass in which there
is much alkaline salt. The construction of _achromatic telescopes_
depends upon this principle.
Not only have different rays of light these different properties with
respect to bodies, so as to be more or less refracted, or dispersed, by
them, but different sides of the same rays seem to have different
properties, for they are differently affected on entering a piece of
_island crystal_. With the same degree of incidence; part of the pencil
of rays, consisting of all the colours, proceeds in one direction, and
the rest in a different one; so that objects seen through a piece of
this substance appear double.
At the surface of all bodies rays of light are promiscuously reflected,
or transmitted.
But if the next surface be very near to it, the rays of one colour
chiefly are reflected, and the rest transmitted, and these places occur
alternately for rays of each of the colours in passing from the thinnest
to the thickest parts of the medium; so that several series, or orders,
of colours will be visible on the surface of the same thin transparent
body. On this principle coloured rings appear between a plane and a
convex lens, in a little oil on the surface of water, and in bubbles
made with soap and water.
When rays of light pass near to any body, so as to come within the
sphere of its attraction and repulsion, an _inflection_ takes place; all
the kinds of rays being bent _towards_, or _from_, the body, and these
powers affecting some rays more than others, they are by this means also
separated from each other, so that coloured streaks appear both within
the shadow, and the outside of it, the red rays being inflected at the
greatest distance from the body.
Part of the light which enters bodies is retained within them, and
proceeds no farther; but so loosely in some kinds of bodies, that a
small degree of heat is sufficient to expel it again, so as to make the
body visible in the dark: but the more heat is applied, the sooner is
all the light expelled. This is a strong argument for the materiality of
light. _Bolognian phosphorus_ is a substance which has this property;
but a composition made by Mr. Canton, of calcined oyster-shells and
sulphur, in a much greater degree. However, white paper, and most
substances, except the metals, are possessed of this property in a small
degree.
Some bodies, especially phosphorus, and animal substances tending to
putrefaction, emit light without being sensibly hot.
The _colours_ of vegetables, and likewise their _taste_ and _smell_,
depend upon light. It is also by means of light falling on the leaves
and other green parts of plants, that they emit dephlogisticated air,
which preserves the atmosphere fit for respiration.
It is light that imparts colour to the skins of men, by means of the
fluid immediately under them. This is the cause of _tanning_, of the
_copper colour_ of the North Americans, and the _black_ of the Negroes.
Light also gives colour to several other substances, especially the
solutions of mercury in acids.