_Light._
Besides the hearing and sound there is another mode by which we
become sensible of the impressions of external objects, namely,
sight and light. This subject also offers some observations bearing
on our present purpose.
It has been declared by writers on Natural Theology, that the
human eye exhibits such evidence of design and skill in its
construction, that no one, who considers it attentively, can resist
this impression: nor does this appear to be saying too much. It
must, at the same time, be obvious that this construction of the
eye could not answer its purposes, except the constitution of light
corresponded to it. Light is an element of the most peculiar kind
and properties, and such an element can hardly be conceived to have
been placed in the universe without a regard to its operation and
functions. As the eye is made for light, so light must have been
made, at least among other ends, for the eye.
1. We must expect to comprehend imperfectly only the mechanism
of the elements. Still, we have endeavoured to show that in some
instances the arrangements by which their purposes are effected
are, to a certain extent, intelligible. In order to explain,
however, in what manner light answers those ends which appear to us
its principal ones, we must know something of the nature of light.
There have, hitherto, been, among men of science, two prevailing
opinions upon this subject: some considering light as consisting
in the emission of luminous particles; others accounting for
its phenomena by the propagation of vibrations through a highly
subtle and elastic _ether_. The former opinion has, till lately,
been most generally entertained in this country, having been the
hypothesis on which Newton made his calculations; the latter is
the one to which most of those persons have been led, who, in
recent times, have endeavoured to deduce general conclusions from
the newly discovered phenomena of light. Among these persons, the
_theory of undulations_ is conceived to be established in nearly
the same manner, and almost as certainly, as the doctrine of
universal gravitation; namely, by a series of laws inferred from
numerous facts, which, proceeding from different sets of phenomena,
are found to converge to one common view; and by calculations
founded upon the theory, which, indicating new and untried facts,
are found to agree exactly with experiment.
We cannot here introduce a sketch of the progress by which the
phenomena have thus led to the acceptance of the theory of
undulations. But this theory appears to have such claims to our
assent, that the views which we have to offer with regard to the
design exercised in the adaptation, of light to its purposes, will
depend on the undulatory theory, so far as they depend on theory at
all.[15]
2. The impressions of sight, like those of hearing, differ in
intensity and in kind. _Brightness_ and _Colour_ are the principal
differences among visible things, as loudness and pitch are among
sounds. But there is a singular distinction between these senses
in one respect: every object and part of an object seen, is
necessarily and inevitably referred to some _position_ in the space
before us; and hence visible things have place, magnitude, form,
as well as light, shade, and colour. There is nothing analogous
to this in the sense of hearing; for though we can, in some
approximate degree, _guess_ the situation of the point from which
a sound proceeds, this is a secondary process, distinguishable from
the perception of the sound itself; whereas we cannot conceive
visible things without form and place.
The law according to which the sense of vision is thus affected,
appears to be this. By the properties of light, the external scene
produces, through the transparent parts of the eye, an image or
picture exactly resembling the reality, upon the back part of the
retina: and each point which we see, is seen in the direction of a
line passing from its image on the retina, through the centre of
the pupil of the eye.[16] In this manner we perceive by the eye the
situation of every point, at the same time that we perceive its
existence; and by combining the situations of many points, we have
forms and outlines of every sort.
That we should receive from the eye this notice of the position
of the object as well as of its other visible qualities, appears
to be absolutely necessary for our intercourse with the external
world; and the faculty of doing so is so intimate a part of our
constitution that we cannot conceive ourselves divested of it.
Yet in order to imagine ourselves destitute of this faculty, we
have only to suppose that the eye should receive its impressions
as the ear does, and should apprehend red and green, bright and
dark, without placing them side by side; as the ear takes in the
different sounds which compose a concert, without attributing them
to different parts of space.
The peculiar property thus belonging to vision, of perceiving
position, is so essential to us, that we may readily believe that
some particular provision has been made for its existence. The
remarkable mechanism of the eye (precisely resembling that of a
_camera obscura_,) by which it produces an image on the nervous
web forming its hinder part, seems to have this effect for its
main object. And this mechanism necessarily supposes certain
corresponding properties in light itself, by means of which such an
effect becomes possible.
The main properties of light which are concerned in this
arrangement, are _reflexion_ and _refraction_: reflexion by which
light is reflected and scattered by all objects, and thus comes to
the eye from all: and refraction, by which its course is bent, when
it passes obliquely out of one transparent medium into another;
and by which, consequently, convex transparent substances, such as
the cornea and humours of the eye, possess the power of making the
light converge to a _focus_ or point; an assemblage of such points
forming the images on the retina, which we have mentioned.
Reflexion and refraction are therefore the essential and
indispensable properties of light; and so far as we can understand,
it appears that it was necessary that light should possess such
properties, in order that it might form a medium of communication
between man and the external world. We may consider its power of
passing through transparent media (as air) to be given in order
that it may enlighten the earth; its affection of reflexion, for
the purpose of making colours visible; and its refraction to
be bestowed, that it may enable us to discriminate figure and
position, by means of the lenses of the eye.
In this manner light may be considered as constituted with a
peculiar reference to the eyes of animals, and its leading
properties may be looked upon as contrivances or adaptations to
fit it for its visual office. And in such a point of view the
perfection of the contrivance or adaptation must be allowed to be
very remarkable.
3. But besides the properties of reflexion and refraction, the most
obvious laws of light, an extraordinary variety of phenomena have
lately been discovered, regulated by other laws of the most curious
kind, uniting great complexity with great symmetry. We refer to the
phenomena of diffraction, polarisation, and periodical colours,
produced by crystals and by thin plates. We have, in these facts,
a vast mass of properties and laws, offering a subject of study
which has been pursued with eminent skill and intelligence. But
these properties and laws, so far as has yet been discovered,
exert no agency whatever, and have no purpose, in the general
economy of nature. Beams of light polarised in contrary directions
exhibit the most remarkable differences when they pass through
certain crystals, but manifest no discoverable difference in their
immediate impression on the eye. We have, therefore, here, a number
of laws of light, which we cannot perceive to be established with
any design which has a reference to the other parts of the universe.
Undoubtedly it is exceedingly possible that these differences of
light may operate in some quarter, and in some way, which we cannot
detect; and that these laws may have purposes and may answer ends
of which we have no suspicion. All the analogy of nature teaches
us a lesson of humility, with regard to the reliance we are to
place on our discernment and judgment as to such matters. But with
our present knowledge, we may observe, that this curious system of
phenomena appears to be a collateral result of the mechanism by
which the effects of light are produced; and therefore a necessary
consequence of the existence of that element of which the offices
are so numerous and so beneficent.
The new properties of light, and the speculations founded upon
them, have led many persons to the belief of the undulatory theory;
which, as we have said, is considered by some philosophers as
demonstrated. If we adopt this theory, we consider the luminiferous
ether to have no local motion; and to produce refraction and
reflexion by the operation of its elasticity alone. We must
necessarily suppose the tenuity of the ether to be extreme; and
if we moreover suppose its tension to be very great, which the
vast velocity of light requires us to suppose, the vibrations by
which light is propagated will be _transverse_ vibrations, that
is the motion to and fro will be athwart the line along which the
undulation travels; and from this circumstance all the laws of
polarisation necessarily follow. And the properties of transverse
vibrations, combined with the properties of vibrations in general,
give rise to all the curious and numerous phenomena of colours of
which we have spoken.
If the vibrations be transverse, they may be resolved into two
different planes; this is _polarisation_: if they fall on a
medium which has different elasticity in different directions,
they will be divided into two sets of vibrations; this is _double
refraction_; and so on. Some of the new properties, however, as the
fringes of shadows and the colours of thin plates, follow from the
undulatory theory, whether the vibrations be transverse or not.
It would appear, therefore, that the propagation of light by
means of a subtle medium, leads necessarily to the extraordinary
collection of properties which have recently been discovered; and,
at any rate, its propagation by the transverse vibrations of such a
medium does lead inevitably to these results.
Leaving it therefore to future times to point out the other reasons
(or _uses_ if they exist) of these newly discovered properties of
light, in their bearing on other parts of the world, we may venture
to say, that if light was to be propagated through transparent
media by the undulations of a subtle fluid, these properties must
result, as necessarily as the rainbow results from the unequal
refrangibility of different colours. This phenomenon and those,
appear alike to be the collateral consequences of the law’s
impressed on light with a view to its principal offices.
Thus the exquisitely beautiful and symmetrical phenomena and laws
of polarisation, and of crystalline and other effects, may be
looked upon as indications of the delicacy and subtlety of the
mechanism by which man, through his visual organs, is put in
communication with the external world; is made acquainted with the
forms and qualities of objects in the most remote regions of space;
and is enabled, in some measure, to determine his position and
relation in a universe in which he is but an atom.
4. If we suppose it clearly established that light is produced
by the vibrations of an ether, we find considerations offer
themselves, similar to those which occurred in the case of sound.
The vibrations of this ether affect our organs with the sense of
light and colour. Why, or how do they do this? It is only within
certain limits that the effect is produced, and these limits are
comparatively narrower here than in the case of sound. The whole
scale of colour, from violet to crimson, lies between vibrations
which are four hundred and fifty-eight million millions, and seven
hundred and twenty-seven million millions in a second; a proportion
much smaller than the corresponding ratio for perceptible sounds.
Why should such vibrations produce perception in the eye, and
no others? There must be here some peculiar adaptation of the
sensitive powers to these wonderfully minute and condensed
mechanical motions. What happens when the vibrations are slower
than the red, or quicker than the blue? They do not produce vision:
do they produce any effect? Have they any thing to do with heat or
with electricity? We cannot tell. The ether must be as susceptible
of these vibrations, as of those which produce vision. But the
mechanism of the eye is adjusted to this latter kind only; and this
precise kind, (whether alone or mixed with others,) proceeds from
the sun and from other luminaries, and thus communicates to us the
state of the visible universe. The mere material elements then are
full of properties which we can understand no otherwise, than as
the results of a refined contrivance.