LIGHT AND OPTICS.
Light—Optics—Solar Spectrum—Decomposition of Light—Light, Heat, and
Actinism—Blue Paper and Color for the Walls of the Operating
Room—Proportions of Light, Heat, and Actinism composing a
Sunbeam—Refraction—Reflection—Lenses—Copying Spherical
Aberration—Chromatic Aberration.
It is advisable that persons engaging in the Daguerreotype art should
have at least a little knowledge of the general principles of light and
optics. It is not the author’s design here to give a full treatise on
these subjects, but he only briefly refers to the matter, giving a few
facts.
It has been well observed by an able writer, that it is impossible to
trace the path of a sunbeam through our atmosphere without feeling a
desire to know its nature, by what power it traverses the immensity of
space, and the various modifications it undergoes at the surfaces and
interior of terrestrial substances.
Light is white and colorless, as long as it does not come in contact
with matter. When in apposition with any body, it suffers variable
degrees of decomposition, resulting in color, as by reflection,
dispersion, refraction, and unequal absorption.
To Sir I. Newton the world is indebted for proving the compound nature
of a ray of white light emitted from the sun. The object of this work
is not to engage in an extended theory upon the subject of light, but
to recur only to some points of more particular interest to the
photographic operator.
The decomposition of a beam of light can be noticed by exposing it to a
prism. If, in a dark room, a beam of light be admitted through a small
hole in a shutter, it will form a white round spot upon the place where
it falls. If a triangular prism of glass be placed on the inside of the
dark room, so that the beam of light falls upon it, it no longer has
the same direction, nor does it form a round spot, but an oblong
painted image of seven colors—red, orange, yellow, green, blue, indigo,
and violet. This is called the solar spectrum, and will be readily
understood by reference to the accompanying diagram, Fig. 1.
{133}
To those who are unacquainted with the theory of light (and for their
benefit this chapter is given), it may be a matter of wonder how a beam
of light can be divided.
[Illustration: Fig. 1]
This can be understood when I say, that white light is a bundle of
colored rays united together, and when so incorporated, they are
colorless; but in passing through the prism the bond of union is
severed, and the colored rays come out singly and separately, because
each ray has a certain amount of refracting (bending) power, peculiar
to itself. These rays always hold the same relation to each other, as
may be seen by comparing every spectrum or rainbow; there is never any
confusion or misplacement.
There are various other means of decomposing {134} white light besides
the prism, of which one of the principal and most interesting to the
Daguerreotypist is by reflection from colored bodies. If a beam of
white light falls upon a white surface, it is reflected without change;
but if it falls upon a red surface, only the red ray is reflected: so
also with yellow and other colors. The ray which is reflected
corresponds with the color of the object. It is this reflected
decomposed light which prevents the beautifully-colored image we see
upon the ground glass in our cameras.
[Illustration: Fig. 2]
A sunbeam may be capable of three divisions—LIGHT, HEAT, and ACTINISM;
the last causes all the chemical changes, and is the acting power upon
surfaces prepared to receive the photographic image. The accompanying
illustration, Fig. 2, will readily bring to the mind of the reader the
relation of these one to another, and their intensities in the
different parts of a decomposed sunbeam.
The various points of the solar spectrum are represented in the order
in which they occur between A, and B, this exhibits the limits of the
Newtonian spectrum, corresponding with Fig. 1. Sir John Herschel and
Seebeck have shown that there exists, beyond the violet, a faint violet
light, or rather a lavender to b, to which gradually becomes colorless;
similarly, red light exists beyond the assigned limits of the red ray
to a. The greatest amount of actinic power is shown at E opposite the
violet; hence this color “exerts” the greatest amount of influence in
the formation of the photographic image.
(Blue paper and blue color have been somewhat extensively used by our
Daguerreotype operators in their operating rooms and skylights, in
order to facilitate the operation in the camera. I fancy, however, that
this plan cannot be productive of as much good as thought by some, from
the fact, that the light falling upon the subject, and then reflected
into the camera, is, coming through colorless glass, not affected by
such rays as may be reflected from the walls of the operating room; and
even if it were so, I conceive that it would be injurious, by
destroying the harmony of shadows which might otherwise occur.) The
greatest amount of white light is at C; the yellow contains less of the
chemical power than any other portion of the solar spectrum. It has
been found that the most intense heat is at the extreme red, b.
Artificial lights differ in their color; the white light of burning
charcoal, which is the principal light from candles, oil and gas,
contains three rays—red, yellow, and blue. The dazzling light emitted
from lime intensely heated, known as the Drummond light, gives the
colors of the prism almost as bright as the solar spectrum.
If we expose a prepared Daguerreotype plate or sensitive paper to the
solar spectrum, it will be observed that the luminous power (the
yellow) occupies but a small space compared with the influence of heat
and chemical power. R. Hunt, in his Researches on Light, has presented
the following remarks upon the accompanying illustration:
[Illustration: Fig. 3]
“If the linear measure, or the diameter of a circle which shall include
the luminous rays, is 25, that of the calorific spectrum will be 42.10,
and of the chemical spectrum 55.10. Such a series of circles may well
be used to represent a beam from the sun, which may be regarded as an
atom of Light, surrounded with an invisible atmosphere of Heat, and
another still more extended, which possesses the remarkable property of
producing chemical and molecular change.
A ray of light, in passing obliquely through any medium of uniform
density, does not change its course; but if it should pass into a
denser body, it would turn from a straight line, pursue a less oblique
direction, and in a line nearer to a perpendicular to the surface of
that body. Water exerts a stronger refracting power than air; and if a
ray of light fall upon a body of this fluid its course is changed, as
may be seen by reference to Fig. 4.
[Illustration: Fig. 4]
It is observed that it proceeds in a less oblique direction (towards
the dotted line), and, on passing on through, leaves the liquid,
proceeding in a line parallel to that at which it entered. It should be
observed that at the surface of bodies the refractive power is exerted,
and that the light proceeds in a straight line until leaving the body.
The refraction is more or less, and in all cases in proportion as the
rays fall more or less obliquely on the refracting surface. It is this
law of optics which has given rise to the lenses in our camera tubes,
by which means we are enabled to secure a well-delineated
representation of any object we choose to picture.
When a ray of light passes from one medium to another, and through that
into the first again, if the two refractions be equal, and in opposite
directions, no sensible effect will be produced.
The reader may readily comprehend the phenomena of refraction, by means
of light passing through lenses of different curves, by reference to
the following diagrams:—
[Illustration: Fig. 5, 6, 7]
Fig 5 represents a double-convex lens, Fig. 6 a double-concave, and
Fig. 7 a concavo-convex or meniscus. By these it is seen that a
double-convex lens tends to condense the rays of light to a focus, a
double-concave to scatter them, and a concavo-convex combines both
powers.
If parallel rays of light fall upon a double-convex lens, D D, Fig. 8,
they will be refracted (excepting such as pass directly through the
centre) to a point termed the principal focus.
[Illustration: Fig. 8]
The lines A B C represent parallel rays which pass through the lens, D
D, and meet at F; this point being the principal focus, its distance
from the lens is called the focal length. Those rays of light which are
traversing a parallel course, when they enter the lens are brought to a
focus nearer the lens than others. Hence the difficulty the operator
sometimes experiences by not being able to “obtain a focus,” when he
wishes to secure a picture of some very distant objects; he does not
get his ground glass near enough to the lenses. Again, the rays from an
object near by may be termed diverging rays. This will be better
comprehended by reference to Fig. 9, where it will be seen that the
dotted lines, representing parallel rays, meet nearer the lenses than
those from the point A. The closer the object is to the lenses, the
greater will be the divergence. This rule is applicable to copying. Did
we wish to copy a 1/6 size Daguerreotype on a 1/16 size plate, we
should place it in such a position to the lenses at A that the focus
would be at F, where the image would be represented at about the proper
size. Now, if we should wish to copy the 1/6 size picture, and produce
another of exactly the same dimensions, we have only to bring it nearer
to the lenses, so that the lens D E shall be equi-distant from the
picture and the focus, i. e. from A to B. The reason of this is, that
the distance of the picture from the lens, in the last copy, is less
than the other, and the divergence has increased, throwing, the focus
further from the lens.”
[Illustration: Fig. 9]
These remarks have been introduced here as being important for those
who may not understand the principles of enlarging or reducing pictures
in copying.
I would remark that the points F and A, in Fig. 9, are termed
“conjugate foci.”
If we hold a double-convex lens opposite any object, we find that an
inverted image of that object will be formed on a paper held behind it.
To illustrate this more clearly, I will refer to the following woodcut:
[Illustration: Fig. 10]
“If A B C is an object placed before a convex lens, L L, every point of
it will send forth rays in all directions; but, for the sake of
simplicity, suppose only three points to give out rays, one at the top,
one at the middle, and one at the bottom; the whole of the rays then
that proceed from the point A, and fall on the lens L L, will be
refracted and form an image somewhere on the line A G E, which is drawn
direct through the centre of the lens; consequently the focus E,
produced by the convergence of the rays proceding from A, must form an
image of A, only in a different relative position; the middle point of
C being in a direct line with the axis of the lens, will have its image
formed on the axis F, and the rays proceeding from the point B will
form an image at D; so that by imagining luminous objects to be made up
of all infinite number of radiating points and the rays from each
individual point, although falling on the whole surface of the lens, to
converge again and form a focus or representation of that point from
which the rays first emerged, it will be very easy to comprehend how
images are formed, and the cause of those images being reversed.
“It must also be evident, that in the two triangles A G B and D G E,
that E D, the length of the image, must be to A B, the length of the
object, as G D, the distance of the image, is to G B, the distance of
the object from the lens.
It will be observed that in the last cut the image produced by the lens
is curved. Now, it would be impossible to produce a well-defined image
from the centre to the edge upon a plain surface; the outer edges would
be misty, indistinct, or crayon-like. The centre of the image might be
represented clear and sharp on the ground glass, yet this would be far
from the case in regard to the outer portions. This is called spherical
aberration, and to it is due the want of distinctness which is
frequently noticed around the edges of pictures taken in the camera. To
secure a camera with a flat, sharp, field, should be the object of
every operator; and, in a measure, this constitutes the great
difference in cameras manufactured in this country.
Spherical aberration is overcome by proper care in the formation of the
lens: “It can be shown upon mathematical data that a lens similar to
that given in the following diagram—one surface of which is a section
of an ellipse, and the other of a circle struck from the furthest of
the two foci of that ellipse—produces no aberration.
“At the earliest period of the employment of the camera obscura, a
double-convex lens was used to produce the image; but this form was
soon abandoned, on account of the spherical aberration so caused.
Lenses for the photographic camera are now always ground of a
concavo-convex form, or meniscus, which corresponds more nearly to the
accompanying diagram.”
[Illustration: Fig. 11]
Chromatic Aberration is another difficulty that opticians have to
contend with in the manufacturing of lenses. It will be remembered,
that in a former page (133) a beam of light is decomposed by passing
through a glass prism giving seven distinct colors—red, orange, yellow,
green, blue, indigo and violet.
Now, as has been said before, the dissimilar rays having an unequal
degree of refrangibility, it will be impossible to obtain a focus by
the light passing through a double-convex lens without its being
fringed with color. Its effect will be readily understood by reference
to the accompanying cut.
[Illustration: Fig. 8]
If L L be a double convex-lens, and R R R parallel rays of white light,
composed of the seven colored rays, each having a different index of
refraction, they cannot be refracted to one and the same point; the red
rays, being the least refrangible, will be bent to r, and the violet
rays, being the most refrangible, to v: the distance v r constitutes
the chromatic aberration, and the circle, of which the diameter is a l,
the place or point of mean refraction, and is called the circle of
least aberration. If the rays of the sun are refracted by means of a
lens, and the image received on a screen placed between C and o, so as
to cut the cone L a l L, a luminous circle will be formed on the paper,
only surrounded by a red border, because it is produced by a section of
the cone L a l L, of which the external rays L a L l, are red; if the
screen be moved to the other side of o, the luminous circle will be
bordered with violet, because it will be a section of the cone M a M l,
of which the exterior rays are violet. To avoid the influence of
spherical aberration, and to render the phenomena of coloration more
evident, let an opaque disc be placed over the central portion of the
lens, so as to allow the rays only to pass which are at the edge of the
glass; a violet image of the sun will then be seen at v, red at r, and,
finally, images of all the colors of the spectrum in the intermediate
space; consequently, the general image will not only be confused, but
clothed with prismatic colors.”
To overcome the difficulty arising from the chromatic aberration, the
optician has only to employ a combination of lenses of opposite focal
length, and cut from glass possessing different refrangible powers, so
that the rays of light passing through the one are strongly refracted,
and in the other are bent asunder again, reproducing white light.
To the photographer one of the most important features, requiring his
particular attention, is, that he be provided with a good lens. By the
remarks given in the preceding pages, he will be enabled, in a measure,
to judge of some of the difficulties to which he is occasionally
subjected. We have in this country but two or three individuals who are
giving their attention to the manufacture of lenses, and their
construction is such, that they are quite free from the spherical or
chromatic aberration.