Some philosophers contend that to the existence of light alone we owe
the beautiful effects produced by the Photogenic art, while others give
sufficient reasons for doubting the correctness of the assumption.
That the results are effected by a principle associated with light and
not by the luminous principle itself, is the most probable conclusion.
The importance of a knowledge of this fact becomes most essential in
practice, as will presently be seen. To this principle Mr. Hunt gives
the name of ENERGIA.
THE NATURE of Light is not wholly known, but it is generally believed
to be matter, as in its motions it obeys the laws regulating matter.
So closely is it connected with heat and electricity that there can be
little doubt of their all being but different modifications of the same
substance. I will not, however, enter into a statement of the various
theories of Philosophers on this head, but content myself with that of
Sir Isaac Newton; who supposed rays of light to consist of minute
particles of matter, which are constantly emanating from luminous
bodies and cause vision, as odoriferous particles, proceeding from
certain bodies, cause smelling.
The effects of light upon other bodies, and how light is effected by
them, involve some of the most important principles, which if properly
understood by Daguerreotypists would enable them to improve and correct
many of the practical operations in their art. These effects we shall
exhibit in this and the following chapters. Before we enter on this
subject it will be necessary to become familiar with the
DEFINITIONS of some of the terms used in the science of optics.
Luminous bodies are of two kinds; those which shine by their own light,
and those which shine by reflected light.
Transparent bodies are such as permit rays of light to pass through
them.
Translucent bodies permit light to pass faintly, but without
representing the figure of objects seen through them.
Opaque bodies permit no light to pass through them, but reflect light.
A ray is a line of light.
A beam is a collection of parallel rays.
A pencil is a collection of converging, or diverging rays.
A medium is any space through which light passes.
Incident rays are those which fall upon the surface of a body.
Reflected rays are those which are thrown off from a body.
Parallel rays are such as proceed equally distant from each other
through their whole course.
Converging rays are such as approach and tend to unite at any one
point, as at b. Fig. 3.
Diverging rays are those which continue to recede from each other, as
at e. Fig. 3.
A Focus is that point at which converging rays meet.
MOTION OF LIGHT--Rays of light are thrown off from luminous bodies in
every direction, but always in straight lines, which cross each other
at every point; but the particles of which each ray consists are so
minute that the rays do not appear to be impeded by each other. A ray
of light passing through an aperture into a dark room, proceeds in a
straight line; a fact of which any one may be convinced by going into a
darkened room and admitting light only through a small aperture.
[Illustration: Fig. 1 (hipho_1.gif)]
Light also moves with great velocity, but becomes fainter as it recedes
from the source from which it eminates; in other words, diverging rays
of light diminish in intensity as the square of the distance increases.
For instance let a fig. 1, represent the luminous body from which light
proceeds, and suppose three square boards, b. c. d. severally one,
four and sixteen square inches in size be placed; b one foot, c two
feet, and d four feet from a, it will be perceived that the smallest
board b will throw c into shadow; that is, obstruct all rays of light
that would otherwise fall on c, and if b were removed c would in like
manner hide the light from d--Now, if b recieve as much light as would
fall on c whose surface is four times as large, the light must be four
times as powerful and sixteen times as powerful as that which would
fall on the second and third boards, because the same quantity of light
is diffused over a space four and sixteen times greater. These same
rays may be collected and their intensity again increased.
Rays of light are reflected from one surface to another; Refracted, or
bent, as they pass from the surface of one transparent medium to
another; and Inflected, or turned from their course, by the attraction
of opaque bodies. From the first we derive the principles on which
mirrors are constructed; to the second we are indebted for the power of
the lenses, and the blessings of sight,--for the light acts upon the
retina of the eye in the same manner as on the lens of a camera. The
latter has no important bearing upon our subject.
When a ray of light falls perpendicularly upon an opaque body, it is
reflected bark in the same line in which it proceeds; in this case the
reflected ray returns in the same path the incident ray traversed; but
when a ray falls obliquely, it is reflected obliquely, that is, it is
thrown off in opposite direction, and as far from the perpendicular as
was the incident ray, as shown at Fig. 2; a representing the incident
ray and b the reflected. The point, or angle c made by the incident
ray, at the surface of the reflector e f, with a line c d,
perpendicular to that surface, is called the angle of incidence, while
the angle formed by the reflected ray b and the perpendicular line d is
called the angle of reflection, and these angles are always equal.
[Illustration: Fig. 2 (hipho_2.gif)]
It is by this reflection of light that objects are made visible; but
unless light falls directly upon the eye they are invisible, and are
not sensibly felt until after a certain series of operations upon the
various coverings and humors of the eye. Smooth and polished surfaces
reflect light most powerfully, and send to the eye the images of the
objects from which the light proceeded before reflection. Glass, which
is transparent--transmitting light--would be of no use to us as a
mirror, were it not first coated on one side with a metalic amalgam,
which interrupts the rays in their passage from the glass into the air,
and throws them either directly in the incident line, or in an oblique
direction. The reason why trees, rocks and animals are not all
mirrors, reflecting other forms instead of their own, is, that their
surfaces are uneven, and rays of light reflected from an uneven surface
are diffused in all directions.
Parallel rays falling obliquely upon a plane mirror are reflected
parallel; converging rays, with the same degree of convergence; and
diverging rays equally divergent.
Stand before a mirror and your image is formed therein, and appears to
be as far behind the glass as you are before it, making the angle of
reflection equal to that of incidence, as before stated. The incident
ray and the reflected ray form, together, what is called the passage of
reflection, and this will therefore make the actual distance of an
image to appear as far again from the eye as it really is. Any object
which reflects light is called a radiant. The point behind a
reflecting surface, from which they appear to diverge, is called the
virtual focus.
Rays of light being reflected at the same angle at which they fall upon
a mirror, two persons can stand in such a position that each can see
the image of the other without seeing his own. Again; you may see your
whole figure in a mirror half your length, but if you stand before one
a few inches shorter the whole cannot be reflected, as the incident ray
which passes from your feet into the mirror in the former case, will in
the latter fall under it. Images are always reversed in mirrors.
Convex mirrors reflect light from a rounded surface and disperse the
rays in every direction, causing parallel rays to diverge, diverging
rays to diverge more, and converging rays to converge less--they
represent objects smaller than they really are--because the angle
formed by the reflected ray is rendered more acute by a convex than by
a plane surface, and it is the diminishing of the visual angle, by
causing rays of light to be farther extended before they meet in a
point, which produces the image of convex mirrors. The greater the
convexity of a mirror, the more will the images of the objects be
diminished, and the nearer will they appear to the surface. These
mirrors furnish science with many curious and pleasing facts.
Concave mirrors are the reverse of convex; the latter being rounded
outwards, the former hollowed inwards--they render rays of light more
converging--collect rays instead of dispersing them, and magnify
objects while the convex diminishes them.
Rays of light may be collected in the focus of a mirror to such
intensity as to melt metals. The ordinary burning glass is an
illustration of this fact; although the rays of light are refracted, or
passed through the glass and concentrated into a focus beneath.
When incident rays are parallel, the reflected rays converge to a
focus, but when the incident rays proceed from a focus, or are
divergent, they are reflected parallel. It is only when an object is
nearer to a concave mirror than its centre of concavity, that its image
is magnified; for when the object is farther from the mirror, this
centre will appear less than the object, and in an inverted position.
The centre of concavity in a concave mirror, is an imaginary point
placed in the centre of a circle formed by continuing the boundary of
the concavity of the mirror from any one point of the edge to another
parallel to and beneath it.
REFRACTION OF LIGHT:--I now pass to the consideration of the passage of
light through bodies.
A ray of light failing perpendicularly through the air upon a surface
of glass or water passes on in a straight line through the body; but if
it, in passing from one medium to another of different density, fall
obliquely, it is bent from its direct course and recedes from it,
either towards the right or left, and this bending is called
refraction; (see Fig. 3, b.) If a ray of light passes from a rarer into
a denser medium it is refracted towards a perpendicular in that medium;
but if it passes from a denser into rarer it is bent further from a
perpendicular in that medium. Owing to this bending of the rays of
light the angles of refraction and incidence are never equal.
Transparent bodies differ in their power of bending light--as a general
rule, the refractive power is proportioned to the density--but the
chemical constitution of bodies as well as their density, is found to
effect their refracting power. Inflammable bodies possess this power
to a great degree.
The sines of the angle of incidence and refraction (that is, the
perpendicular drawn from the extremity of an arc to the diameter of a
circle,) are always in the same ratio; viz: from air into water, the
sine of the angle of refraction is nearly as four to three, whatever be
the position of the ray with respect to the refracting surface. From
air into sulphur, the sine of the angle of refraction is as two to
one--therefore the rays of light cannot be refracted whenever the sine
of the angle of refraction becomes equal to the radius* of a circle,
and light falling very obliquely upon a transparent medium ceases to be
refracted; this is termed total reflection.
* The RADIUS of a circle is a straight line passing from the centre to
the circumference.
Since the brightness of a reflected image depends upon the quantity of
light, it is quite evident that those images which arise from total
reflection are by far the most vivid, as in ordinary cases of
reflection a portion of light is absorbed.
I should be pleased to enter more fully into this branch of the science
of optics, but the bounds to which I am necessarily limited in a work
of this kind will not admit of it. In the next chapter, however, I
shall give a synopsis of Mr. Hunt's treatise on the "Influence of the
Solar Rays on Compound Bodies, with especial reference to their
Photographic application"--a work which should be in the hands of every
Daguerreotypist, and which I hope soon to see republished in this
country. I will conclude this chapter with a brief statement of the
principles upon which the Photographic art is founded.
SOLAR and Stellar light contains three kinds of rays, viz: