VISION—THE EYE—THE STEREOSCOPE—SPECTRUM ANALYSIS—THE
SPECTROSCOPE—THE TELESCOPE AND MICROSCOPE—PHOTOGRAPHY—DISSOLVING
VIEWS—LUMINOUS PAINT.
The eye and vision are such important subjects to all of us that
we may be excused for saying something more concerning phenomena
connected with them, and the instruments we use for assisting them.
We do not propose to write a treatise upon the physiology of vision,
for we know the image in the eye is produced physically in the same
manner as the image in a camera obscura. In the eye the sides of the
box are represented by the sclerotic (see chap. x. fig. 95); the dark
inner surface has its parallel in the pigment of the choroid; the
opening in the box in the pupil of the eye; the convex lens in the
crystalline and the cornea; and the retina receives the image. But why
we see—beyond the fact that we do see—no one can explain. Science is
dumb on the subject. Thought and consciousness elude our grasp; and, as
Professor Tyndall says on this subject, “we stand face to face with the
incomprehensible.”
But there are many interesting facts connected with our vision which
may be plainly described. Some people are obliged to carry an object
(or a book) to within ten inches of the eye to see it distinctly; and
a person who does not possess convergent power of the eye will have
to move it farther off, or use convex glasses; while a “near-sighted”
person, whose eyes are too quickly convergent, will use concave glasses
to bring the object near to the eye.
There is but one small place in the retina of the eye which admits
of perfect vision. This, the most sensitive portion, is called the
“yellow spot,” and vision becomes more and more indistinct from this
point towards the circumference. This can be proved by any one; for
in reading we are obliged to carry our eyes from word to word, and
backwards and forwards along the lines of print. Another very important
element in our vision is the contraction and enlargement of the iris
around the pupil. In cases where strong light would only dazzle us
the iris expands, and the pupil is contracted to a sufficient size to
accommodate our vision. At night, or in a darkened room, the pupil is
enlarged. This change will account for our not being immediately able
to see objects when we have passed from darkness to great light, or
_vice versâ_. The iris must have time to accommodate itself to the
light.
Now, outside the small space of perfect vision above mentioned, there
is a circle of considerable extent, called the “field of vision.” In
man this field, when the eyes are fixed, subtends an angle of about
180°, because beyond that the rays cannot enter the pupil of the eye.
But in the lower animals, the fish and birds,—notably the ostrich,—the
field of vision is much more extensive, because the pupils are more
prominent, or the eyes are set more towards the sides of the head. The
ostrich can see behind him, and fish can see in any direction without
apparent limit. Man can only see indistinctly; and though he can move
his eyes rapidly, he can see distinctly but a small portion of any
object at a time, yet he sees with both eyes simultaneously a single
object, because the two lines of vision unite at a single point, and as
the two images cover each other we perceive only one image. Beyond or
within this point of meeting the vision is indistinct, but the angle of
convergence is always varied according to the distance of the object.
If we hold up a penholder in front of us, and in a line with any other
defined object,—say an ink-bottle,—we can see the penholder distinctly,
and the ink-bottle indistinctly, as _two_ images. If we then look at
the ink-bottle we shall see it single, while the penholder will appear
double, but with imperfect outlines.
[Illustration: Fig. 147.—The Stereoscope.]
[Illustration: Fig. 148.—Mode of taking photograph for Stereoscope.]
Again, if we look at a box both eyes will see it equally well, but the
right eye will see a little more on its right side, and the left eye on
the left. It is on this principle that the Stereoscope is constructed.
Sir Charles Wheatstone was the inventor, and the instrument may be
thus described:—Two pictures are taken by photography—one as the
landscape is seen by the right eye, the other as it is viewed by the
left; the points of view thus differing slightly. When both eyes are
simultaneously applied to the instrument the view is seen exactly as
it would appear to the beholder at the actual place it represents. The
views are taken singly; one side at one time, and another after, as in
the camera (fig. 148). A is the first view; B is kept dark; C is the
shutter for A. There are _Reflecting_ and _Refracting_ Stereoscopes.
In the former a mirror reflects the image into each eye; in the latter
the views are pasted on a card, side by side, and looked at through
prismatic lenses. The principles of _Binocular Vision_ have been
applied to the Microscope.
In foregoing chapters we have given many examples with diagrams of the
temporary impressions made upon the retina of the eye. It is a fact
that a wheel revolving at a great rate will appear to be standing still
when suddenly illuminated by a flash of lightning, because the eye has
not time to take in the motion in the instant of time, for the spokes
of the wheel are not moving fast enough to convey the impression of
motion in that half second to the eye; yet the perfect outline of the
wheel is distinctly visible.
Indeed, distinct vision can be exercised in a very small fraction
of a second. It was calculated by Professor Rood, and proved by
experiment, that _forty billionths_ of a second is sufficient time for
the eye to distinguish letters on a printed page. It this instance the
illuminating power was an electric spark from a Leyden Jar.
We have remarked upon the distinctness with which we can see an object
when we direct our gaze upon it, and this appears a self-evident
proposition; but have any of our readers remarked the curious fact
that when they want to see a faint and particular star in the sky
it will at once disappear when they gaze at it? The best way to see
such very faint orbs as this is to _look away from them_,—a little to
one side or the other,—and then the tiny point will become visible
again to the eye. There is also a degree of phosphorescence in the
eye, which any one who receives a blow upon that organ will readily
admit. Even a simple pressure on the closed lid will show us a circle
of light and “colours like a peacock’s tail,” as the great Newton
expressed it. There are many occasions in which light is perceived in
the eye—generally the result of muscular action; and the Irish term to
“knock fire out of my eye” is founded upon philosophical facts.
We are many of us aware of “spots” on our eyes when our digestion is
out of order, and the inability of the eye to see figures distinctly in
a faint light—within a proper seeing distance, too—has often given rise
to the “ghost.” These shadowy forms are nothing more than affections of
the eye, and, as well remarked in Brewster’s Letters on Natural Magic,
“are always _white_ because no other colour can be seen.” The light is
not sufficiently strong to enable the person to see distinctly; and as
the eye passes from side to side, and strives to take in the figure,
it naturally seems shadowy and indistinct, and appears to move as our
eyes move. “When the eye dimly descries an inanimate object whose
different parts reflect different degrees of light, its brighter parts
may enable the spectator to keep up a continued view of it; but the
disappearance and reappearance of its fainter parts, and the change of
shape which ensues, will necessarily give it the semblance of a living
form; and if it occupies a position which is unapproachable, and where
animate objects cannot find their way, the mind will soon transfer to
it a supernatural existence. In like manner a human figure shadowed
forth in a feeble twilight may undergo similar changes, and after being
distinctly seen while it is in a situation favourable for receiving
and reflecting light, it may suddenly disappear in a position before
and within the reach of the observer’s eye; and if this evanescence
takes place in a path or roadway where there is no sideway by which the
figure could escape, it is not easy for an ordinary mind to efface the
impression which it cannot fail to receive.” This will account for many
so-called “ghosts.”
Accidental colours, or ocular spectra, are, so to speak, illusions,
and differently-coloured objects will, when our gaze is turned from
them, give us different “spectra” or images. For instance, a violet
object will, when we turn to a sheet of white paper, give us a
yellow “spectrum”; orange will be blue; black and white will change
respectively; red will become a blue-green. From a very strong white
light the accidental colours will vary.
[Illustration: Fig. 149.—The Solar Spectrum.]
The Solar Spectrum is the name given to the coloured band formed by the
decomposition of a beam of light into its elementary colours, of which
there are seven. This is an easy experiment. A ray of light can be
admitted into a darkened room through a hole in the shutter, and thus
admitted will produce a white spot on the screen opposite, as at _g_ in
the diagram (fig. 149). If we interpose a prism—a triangular piece of
glass—the “drop” of a chandelier will do—we cause it to diverge from
its direct line, and it will produce a longer streak of light lower
down. This streak will exhibit the prismatic colours, or the “colours
of the rainbow”; viz., red (at the top), orange, yellow, green, blue,
indigo (blue), and violet last. These are the colours of the Solar
Spectrum. The white light is thus decomposed, and it is called mixed
light, because of the seven rays of which it is composed. These rays
can be again collected and returned to the white light by means of a
convex lens.
“White light,” said Sir Isaac Newton, “is composed of rays differently
refrangible,” and as we can obtain the colours of the rainbow from
white light, we can, by painting them on a circular plate and turning
it rapidly round, make the plate appear white. Thus we can prove that
the seven colours make “white” when intermingled. But Newton (1675) did
not arrive at the great importance of his experiment. He made a round
hole in the shutter, and found that the various colours overlapped each
other. But, in 1802, Dr. Wollaston improved on this experiment, and by
admitting the light through a tiny slit in the wood, procured an almost
perfect spectrum of “simple” colours, each one perfectly distinct and
divided by black lines.
But twelve years later, Professor Fraunhofer made a chart of these
lines, which are still known by his name. Only, instead of the 576
he discovered, there are now thousands known to us! To Fraunhofer’s
telescope Mr. Simms added a collimating lens, and so the Spectroscope
was begun; and now we use a number of prisms and almost perfect
instruments, dispersing the light through each. We have here an
illustration of a simple Spectroscope, which is much used for chemical
analysis (fig. 150).
In the spectrum we have long and short waves of light, as we have
long and short (high and low) waves in music, called notes. The long
or low notes are as the red rays, the high notes as the blue waves of
light. (Here we have another instance of the similarity between light
and sound.) But suppose we shut out the daylight and substitute an
artificial light. If we use a lamp burning alcohol with salt (chloride
of sodium), the spectrum will only consist of two yellow bands, all the
other colours being absent. With lithium we obtain only two, one orange
and one red. From this we deduce the fact that different substances
when burning produce different spectra; and although a solid may (and
platinum will) give all seven colours in its spectrum, others, as we
have seen, will only give us a few, the portion of the spectrum between
the colours being black. Others are continuous, and transversed by
“lines” or narrow spaces devoid of light; such is the spectrum of the
sun, and by careful and attentive calculation and observation we can
get an approximate idea of the matter surrounding the heavenly bodies.
[Illustration: Fig. 150.—The Spectroscope.]
We have said there are lines crossing the spectrum transversely; these
are called Fraunhofer’s lines, after the philosopher who studied
them; they were, however, discovered by Wollaston. These lines are
caused by light from the lower portion of the sun passing through the
metallic vapours surrounding the orb in a state of incandescence, such
as sodium, iron, etc. One of Fraunhofer’s lines, a black double line
known as D in the yellow portion of the spectrum, was known
to occupy the same place as a certain _luminous_ line produced by
sodium compounds in the flame of a spirit lamp. This gave rise to much
consideration, and at length Kirchkoff proved that the sodium vapour
which gives out yellow light can also absorb that light; and this fact,
viz., that every substance, which at a _certain temperature_ emits
light of a certain refrangibility, possesses at that temperature the
power to absorb that same light. So the black lines are now considered
the reversal of luminous lines due to the incandescent vapours by which
the sun is surrounded. Thus the presence of an element can be found
from black or luminous lines, so the existence of terrestrial elements
in celestial bodies has been discovered by means of preparing charts
of the lines of the terrestrial elements, and comparing them with the
lines of stella _spectra_.
We have supposed the beam of light to enter through a slit in the
shutter, and fall upon a screen or sheet. The solar spectrum shown by
the passage of the beam through a prism is roughly as below—
[Illustration: Fig. 151.—Example of the Spectrum.]
Fraunhofer substituted a telescope for the lens and the screen, and
called his instrument a SPECTROSCOPE. He then observed the lines, which
are always in the same position in the solar spectrum. The principal of
them he designated as A, B, C, D, E, F, G, H. The first three are in
the red part of the spectrum; one in the yellow, then one in the green;
F comes between green and blue, G in the indigo-blue, and H in the
violet. But these by no means exhaust the lines now visible. Year by
year the study of Spectrum Analysis has been perfected more and more,
and now we are aware of more than three thousand “lines” existing in
the solar spectrum. The spectra of the moon and planets contain similar
dark lines as are seen in the solar spectrum, but the fixed stars show
different lines. By spectrum analysis we know the various constituents
of the sun’s atmosphere, and we can fix the result of our observations
made by means of the Spectroscope in the photographic camera. By
the more recent discoveries great studies have been made in “solar
chemistry.”
What can we do with the Spectroscope, or rather, What can we _not_ do?
By Spectroscopy we can find out, and have already far advanced upon
our path of discovery, “the measure of the sun’s rotation, the speed
and direction of the fierce tornados which sweep over its surface, and
give rise to the ‘maelstroms’ we term ‘sunspots,’ and the mighty alps
of glowing gas that shoot far beyond the visible orb, ever changing
their form and size; even the temperature and pressure of the several
layers and their fluctuations are in process of being defined and
determined.” This is what science is doing for us, and when we have
actually succeeded in ascertaining the weather at various depths in
the atmosphere of the sun, we shall be able to predict our own, which
depends so much upon the sun. Last year (1880) Professor Adams, in his
address to the British Association, showed that magnetic disturbances,
identical in kind, took place at places widely apart simultaneously.
He argues that the cause of these identical disturbances must be far
removed from the earth.
“If,” he says, “we imagine the masses of iron, nickel, and magnesium
in the sun to retain even in a slight degree their magnetic power in a
gaseous state, we have a sufficient cause for all our magnetic changes.
We know that masses of metal are ever boiling up from the lower and
hotter levels of the sun’s atmosphere to the cooler upper regions,
where they must again form clouds to throw out their light and heat,
and to absorb the light and heat coming from the hotter lower regions;
then they become condensed, and are drawn back again towards the
body of the sun, so forming those remarkable dark spaces or sunspots
by their down rush to their former levels. In these vast changes we
have abundant cause for those magnetic changes which we observe at
the same instant at distant points on the surface of the earth.” So
we are indebted to the Spectroscope for many wonderful results—the
constitution of the stars, whether they are solid or gaseous, and many
other wonders.
The manner in which we have arrived at these startling conclusions is
not difficult to be understood, but some little explanation will be
necessary.
The existence of dark lines in the solar spectrum proves that certain
rays of solar light are absent, or that there is less light. When we
look through the prism we perceive the spaces or lines, and we can
produce these ourselves by interposing some substance between the
slit in the shutter before mentioned and the prism. The vapour of
sodium will answer our purpose, and we shall find a dark line in the
spectrum, the bright lines being absorbed by the vapour. We can subject
a substance to any temperature we please, and into any condition—solid,
liquid, or gaseous; we can also send the light the substance may give
out through certain media, and we can photograph the spectrum given out
under all conditions. _The distance of the source of the light makes
no difference._ So whether it be the sun, or a far-distant star, we
can tell by the light sent to us what the physical condition of the
star may be. It was discovered in 1864 that the same metallic body
may give different _spectra_; for instance, the spectrum might be a
band of light,—like the rainbow,—or a few isolated colours; or again,
certain detached lines in groups. The brightness of the spectrum lines
will change with the depth of the light-giving source, or matter which
produces it.
We have become aware by means of the Spectroscope that numerous metals
known to us on earth are in combustion in the sun, and new ones have
thus been discovered. In the immense ocean of gas surrounding the
sun there are twenty-two elements as given by Mr. Lockyer, including
iron, sodium, nickel, barium, zinc, lead, calcium, cobalt, hydrogen,
potassium, cadmium, uranium, strontium, etc. Not only is the visible
spectrum capable of minute examination, but, as in the case of the
_heat spectrum_ already mentioned when speaking of Calorescence, the
light spectrum has been traced and photographed far beyond the dark
space after the blue and violet rays, seven times longer than the
visible solar spectrum—a spectrum of light invisible to our finite
vision. Although a telescope has been invented for the examination of
these “ultra violet” rays, no human eye can see them. But—and here
science comes in—when a photographic plate is put in place of the
eye, the tiniest star can be seen and defined. Even the Spectroscope
at length fails, because light at such limits has been held to be
“too coarse-grained for our purposes”! “Light,” says a writer on this
subject “we can then no longer regard as made of smooth rays; we have
to take into account—and to our annoyance—the fact that its ‘long
levelled rules’ are rippled, and its texture, as it were, loose woven”!
Twenty years ago Professors Kirchkoff and Bunsen applied Fraunhofer’s
method to the examination of coloured flames of various substances, and
since then we have been continually investigating the subject; yet much
remains to be learnt of Spectrum Analysis, and Spectroscopy has still
much to reveal. From Newton’s time to the present our scientists have
been slowly but surely examining with the Spectroscope the composition
of _spectra_, and the Spectroscope is now the greatest assistant we
possess.
“Spectrum Analysis, then, teaches us the great fact that solids and
liquids give out continuous spectra, and vapours and gases give out
discontinuous spectra instead of an unbroken light” (Lockyer). We
have found out that the sunlight and moonlight are identical, that
the moon gives us spectrum like a reflection of the former, but has
no atmosphere, and that comets are but gases or vapours. The most
minute particles of a grain of any substance can be detected to the
millionth fraction. The 1/1000 of a grain of blood can be very readily
distinguished in a stain after years have passed. The very year of a
certain vintage of wine has been told by means of “absorption,” or
the action of different bodies on light in the spectrum. It is now
easy, “by means of the absorption of different vapours and different
substances held in solution, to determine not only what the absorbers
really are, but also to detect a minute quantity.” The application of
this theory is due to Dr. Gladstone, who used hollow prisms filled
with certain substances, and so thickened the “absorption lines.” By
these lines, or bands, with the aid of the Spectrum Microscope, most
wonderful discoveries have been made, and will continue to be made. We
will close this portion of the subject with a brief description of the
Spectroscope in principle.
The instrument consists of two telescopes arranged with two
object-glasses on a stand (fig. 150). A narrow slit is put in place
of the eye-piece of one, the arrangement admitting of the slit being
made smaller or larger by means of screws. The glass to which the slit
is attached is called the collimating lens. The light, at the end of
the slit seen from the other telescope, being separated by the prisms
between the two telescopes, will produce the spectrum. The Spectroscope
is enclosed, so that no exterior light shall interfere with the spectra
the student wishes to observe. This merely indicates the principle, not
the details, of the Spectroscope, which vary in different instruments.
We may now pass from the Spectroscope to the Telescope and the
Microscope, instruments to which we are most largely indebted for our
knowledge of our surroundings in earth, air, and water.
The word Telescope is derived from the Greek _tele_, far, and
_skopein_, to see; and the instrument is based upon the property
possessed by a convex lens or concave mirror, of converging to a focus
the rays of light falling on it from any object, and at that point
or focus forming an image of the object. The following diagram will
illustrate this. Let VW be a lens, and AB an object between the glass,
and F the focus. The ray, A_c_, is so refracted as to appear to come
from _a_. The ray from _b_ likewise appears in a similar way, and a
magnified image, _ab_, is the result (fig. 152).
The ordinary Telescope consists of an object-glass and an eye-lens,
with two intermediates to bring the object into an erect position. A
lens brings it near to us, and a magnifier enlarges it for inspection.
We will now give a short history of the Telescope and its improved
construction.
Roger Bacon was supposed to have had some knowledge of the Telescope,
for in 1551 it was written: “Great talke there is of a glass he made
at Oxford, in which men see things that were don.” But a little later,
Baptista Porta found out the power of the convex lens to bring objects
“nearer.” It was, however, according to the old tale, quite by an
accident that the Telescope was discovered about the year 1608.
[Illustration: Fig. 152.—Converging rays to a focus.]
In Middleburg, in Holland, lived a spectacle-maker named Zachary
Jansen, and his sons, when playing with the lenses in the shop,
happened to fix two of them at the proper distance, and then to look
through both. To the astonishment of the boys, they perceived an
inverted image of the church weathercock much nearer and much larger
than usual. They at once told their father what they had seen. He fixed
the glasses in a tube, and having satisfied himself that his sons were
correct, thought little more about the matter. This is the story as
told, but there is little doubt that for the first Telescope the world
was indebted either to Hans Lippersheim or Joseph Adriansz, the former
a spectacle-maker of Middleburg; and in October 1608, Lippersheim
presented to the Government three instruments, with which he “could
see things at a distance.” Jansen came after this. The report of the
invention soon spread, and Galileo, who was then in Venice, eagerly
seized upon the idea, and returning to Padua with some lenses, he
managed to construct a telescope, and began to study the heavens. This
was in 1609. _Galileo’s Tube_ became celebrated, and all the first
telescopes were made with the concave eye-lens. Rheita, a monk, made
a binocular telescope, as now used in our opera- and field-glasses
approximately.
But the prismatic colours which showed themselves in the early
telescopes were not got rid of, nor was it till 1729 that Hall, by
studying the mechanism of the eye, managed a combination of lenses
free from colour. Ten years before (in 1718) Hadley had established
the Reflector Telescope; Herschel made his celebrated forty-foot
“reflector” in 1789.
[Illustration: Fig. 153.—The Microscope.]
However, to resume. In 1747, Euler declared that it was quite possible
to construct an arrangement of lenses so as to obtain a colourless
image, but he was at first challenged by John Dollond. The latter,
however, was afterwards induced to make experiments with prisms of
crown and flint glass. He then tried lenses, and with a concave lens
of flint, and a convex lens of crown, he corrected the colours. The
question of proper curvature was finally settled, and the “Achromatic”
Telescope became an accomplished fact.
There are two classes of Telescopes—the reflecting and refracting. Lord
Rosse’s is an instance of the former. Mr. Grubb’s immense instrument in
Dublin is a refractor.
The Microscope has been also attributed to Zacharias Jansen, and
Drebbel, in 1619, possessed the instrument in London, but it was of
little or no use. The lens invented by Hall, as already mentioned,
gave an impetus to the Microscope. In the simple Microscope the
objects are seen directly through the lens or lenses acting as one.
The compound instrument is composed of two lenses (or a number formed
to do duty as two), an eye-lens, and an object-lens. Between these is
a “stop” to restrain all light, except what is necessary to view the
object distinctly. The large glass near the object bends the rays on
to the eyeglass, and a perfect magnified image is perceived. We annex
diagrams, from which the construction will be readily understood.
[Illustration: Fig. 154.—Image on the Retina.]
We have in the previous chapter mentioned the effect of light upon the
eye and its direction, and when an object is placed very near the eye
we know it cannot be distinctly seen; a magnified image is thrown upon
the retina, and the divergency of the rays prevents a clear image being
perceived. But if a small lens of a short “focal length” be placed in
front of the eye, having PQ for its focus, the rays of light
will be parallel, or very nearly so, and will as such produce “distinct
vision,” and the image will be magnified at _pq_. In the Compound
Refracting Microscope, BAB is the convex lens, near which an
object, PQ, is placed a little beyond its focal length. An
inverted image, _pq_, will then be formed. This image is produced in
the convex lens, _bab′_, and when the rays are reflected out they are
parallel, and are distinctly seen. So the eye of the observer at the
point E will see a magnified image of the object at PQ brought up to
_pq_ (fig. 155).
[Illustration: Fig. 155.—The Microscope lenses.]
Sir Isaac Newton suggested the Reflecting Microscope, and Dr. Wollaston
and Sir David Brewster improved the instrument called the “Periscopic
Microscope,” in which two hemispherical lenses were cemented together
by the plane surfaces, and having a “stop” between them to limit the
aperture. Then the “Achromatic” instrument came into use, and since
then the Microscope has gradually attained perfection.
[Illustration: Fig. 156.—Concave lens.]
We have so frequently mentioned lenses that it may be as well to
say something about them. Lenses may be spherical, double-convex,
plane-convex, plane-concave, double-concave, and concave-convex. Convex
lenses bring the parallel lines which strike them to a focus, as we see
in the “burning-glass.” The concave or hollow lens appears as in fig.
156. The rays that follow it parallel to its axis are refracted, and as
if they came from a point F in the diagram. But converging rays falling
on it emerge in a parallel direction as above, or diverge as in fig.
158.
[Illustration: Fig. 157. 1. Focus of parallel rays. 2. Focus of
divergent rays. 3. Focus of divergent rays brought forward by more
convex lens.]
The use of spectacles to long or short-sighted people is a necessity,
and the lenses used vary. The eye has usually the capacity of suiting
itself to viewing objects—its accommodation, as it is termed—near or
far. But when the forepart of the eye is curved, and cannot adapt
itself to distant objects, the person is said to be short-sighted. In
long sight the axis of the eyeball is too short, and the focus falls
beyond the retina; in short sight it is too long. In the diagrams
herewith fig. 159 shows by the dotted lines the position of the retina
in long sight, and fig. 160 in short sight, the clear lines showing in
each case the perfectly-formed eye. For long sight and old sight the
double-convex glass is used, for short sight the double-concave (fig.
162). We know the burning-glass gives us a small image of the sun as
it converges the rays to its focus. But lenses will do more than this,
and in the PHOTOGRAPHIC CAMERA we find great interest and
amusement.
[Illustration: Fig. 158.—Diverging rays.]
Photography (or writing by light) depends upon the property which
certain preparations possess of being blackened by exposure to light
while in contact with matter. By an achromatic arrangement of lenses
the camera gives us a representation of the desired object Fig. 163
shows the image on the plate, and figs. 164 and 165 the arrangement of
lenses.
[Illustration: Fig. 159.—Hypermetropia (long sight)..]
[Illustration: Fig. 160.—Myopia (short sight).]
[Illustration: Fig. 161.—Concave and convex lenses.]
[Illustration: Fig. 162.—Lenses for long and short sight.]
To Porta, the Neapolitan physician, whose name we have already
mentioned more than once, is due the first idea of the Photographic
Camera. He found that if light was admitted through a small aperture,
objects from which rays reached the hole would be reflected on the
wall like a picture. To this fact we are indebted for the CAMERA
OBSCURA, which receives the picture upon a plane surface by
an arrangement of lenses. In fact, Porta nearly arrived at the
Daguerreotype process. He thought he could teach people to draw by
following the focussed picture with a crayon, but he could not conquer
the aërial perspective.
So the camera languished till 1820, when Wedgwood and Sir Humphrey Davy
attempted to obtain some views with nitrate of silver, but they became
obliterated when exposed to the daylight.
[Illustration: Fig. 163.—The Camera.]
As early as 1814, however, M. Niepce had made a series of experiments
in photography, and subsequently having heard that M. Daguerre was
turning his attention to the same subject, he communicated with
him. In 1827 a paper was read before the Royal Society, and in 1829
a partnership deed was drawn up between Daguerre and Niepce for
“copying engravings by photography.” Daguerre worked hard, and at
length succeeded in obtaining a picture by a long process, to which,
perhaps, some of our readers are indebted for their likenesses forty
years ago. By means of iodine evaporated on a metal plate covered with
“gold-yellow,” and exposing the plate then in a second box to mercurial
vapour, he marked the image in the camera, and then he immersed the
plate in hyposulphate of soda, and was able to expose the image
obtained to daylight.
[Illustration:
Fig. 164. Arrangement of lenses Fig. 165.]
But the mode now in use is the “collodion” process. We have all seen
the photographer pouring the iodized collodion on the plate, and
letting the superfluous liquid drain from a corner of the glass. When
it is dry the glass-plate is dipped into a solution of nitrate of
silver, and then in a few minutes the glass is ready. The focus is
then arranged, and the prepared plate conveyed in a special slide—to
keep it from the light—to the camera. When the “patient” is ready, the
covering of the lens is removed, and the light works the image into
the sensitive plate. The impression is then “brought up,” and when
developed is washed in water, and after by a solution which dissolves
all the silver from the parts not darkened by the light. Thus the
negative is obtained and printed from in the usual manner.
Instantaneous photography is now practised with great success. An
express train, or the movements of a horse at full speed, can thus
be taken in a second or less. These results are obtained by using
prepared plates, and the “emulsion process,” as it is called, succeeds
admirably. The mode of preparation is given in a late work upon the
subject, and the photographic plates may also be obtained ready for
use. Gelatine and water, mixed with bromide of ammonium, nitrate of
silver, and carbonate of ammonium, mixed with certain proportions of
water, form the “emulsion.” We need not go into all the details here.
Information can easily be obtained from published works, and as the
plates can be purchased by amateurs, they will find that the best way.
Aside from the art interest in the new plates is quite another, that
springs from the fact that it is now possible to take pictures of men,
animals, and machinery in rapid motion, thus enabling us to view them
in a way that would be impossible with the unaided eye. The first
experiments in this direction were applied to the movements of a horse
moving at full speed. The pictures, taken in series, showed that he
performed muscular actions that were not before comprehended or even
imagined. These pictures at the time attracted great attention, and
instantaneous pictures have been since taken of dancers in a ball-room,
of vessels and steam-boats in rapid movement, of all kinds of animals
in motion, and of machinery in operation. As the pictures represent the
movements at one instant of time, they give, as it were, a fixed view
of a motion, precisely as if it were suddenly arrested in full action.
In the case of animals, the motions of the nostrils are represented in
the most singular manner, and the spokes of a steam-boat’s paddle-wheel
are shown apparently perfectly still while the spray and waves appear
in active motion, or, rather, as they would look if they could be
instantly frozen. It is clear the new process and pictures will open a
wide and instructive field in art and in the study of mechanical action.
While on the subject of Photography we may mention a very ingenious
little apparatus called a SCENOGRAPH, the invention of Dr.
Candize. It is really a pocket-camera, and is so easily manipulated
that it will be found a most pleasant and useful holiday companion.
Any one may obtain good results with it, and friends of ours have had
occasion to put it in practice during a series of excursions, when it
was found to answer in every instance.
The Scenograph is something like a common Stereoscope in outward
appearance, and would, perhaps, be at first regarded as a mere toy,
did not a more intimate acquaintance prove it a great acquisition,
particularly to explorers and tourists. The tripod stand which supports
the apparatus is, when not in actual use in that capacity, a very
excellent walking-stick, in which the two other “legs” are carried. The
instrument, as will be perceived from the illustration (fig. 166),
is very handy. It produces pictures about the “cabinet” size, and the
whole is so arranged that it can be packed and carried in the pocket
with ease.
[Illustration: Fig. 166.—The Scenograph.]
Photography, as a rule, necessitates a dark room or cabinet, and
many preparations as we all know—a “messing” about with chemicals
and considerable practice before we can become proficient; so it is
not surprising that few amateurs take to it—they prefer to purchase
the pictures. But in the new apparatus of which we are speaking, the
glass plates are already prepared to receive the image. It is not at
all necessary for the operator to stain his fingers and knuckles and
nails with nitrate of silver, or any other “chemicals” whatever. He
just inserts the plate in the Scenograph, and then his apparatus being
steadily set up, he removes the covering from the lens. To develop the
image (in the dusk of the evening or by candle-light) it is necessary
to put some drops of ammonia in a saucer, breathe upon the plate so
as to soften the collodion, and hold it above the ammonia, and then,
under the influence of the vapour, the picture will appear. After this
simple operation the picture will be found fixed for a lengthened
period—practically indefinitely. Thus on the return of the pedestrian
he can reproduce, at small expense, a whole series of little pictures
faithfully representing his holiday tour. The illustration shows a
small apparatus by which on thin plates small photographs can be taken
and fixed till it is found desirable to enlarge them.
The _Photophone_, one of the most recent contributions to science,
is an instrument which, in combination with the telephone principle,
makes it possible to convey sounds by means of a ray of light, and by
means of a quivering beam “to produce articulate speech at a distance.
The success of the Photophone depends upon a rare element, selenium,
which has its “electrical resistance” affected by light. Professor
Adams demonstrated that the resistance of selenium was reduced just
in proportion as the intensity of the light which was acting upon it.
Here was the key to the Photophone as thought out by Professor Bell.
He fancied that he might by means of his telephone produce sound if he
could vary the intensity of the beam of light upon the selenium, which
he connected with his telephone and battery.
The Photophone consists of a transmitter for receiving the voice and
conveying it along the beam of light, and a receiver for taking the
light and converting it into sound—the receiver being the telephone.
There is a small mirror (silvered mica has been used) suspended freely
for vibration. A lens is used to transmit to this the beam of light,
and this beam is again reflected by another lens to the receiver, which
consists of a reflector which has a cell of selenium in its focus,
connected, as already stated, with the telephone and battery. The
speaker stands behind the mirror, and the sound of his voice against
the reverse side makes it vibrate in unison with the sounds uttered.
The movements cause a quivering in the reflected beam, and this in its
changing intensity acts on the selenium, which changes its resistance
accordingly, and through the telephone gives forth a sound!
This is the apparently complicated but really simple, and at the same
time wonderful, invention of Professor Bell. By the Photophone not
only sounds but _movements_ can be converted into sound; even the
_burning of a candle can be heard_! The Photophone is still capable of
improvement, and has not as yet arrived at its full development, for it
is stated it can be made quite independent of a battery or telephone.
There are many phenomena connected with the _Polarization of Light_.
This requires some notice at our hands. We know that a ray of ordinary
light is supposed to be caused by vibrations of the highly attenuated
medium, æther. These vibrations occur across the direction of the
ray; but when they occur only _in one plane_ the light is said to be
“polarized.” Polarization means possessing poles (like a magnet); the
polarized rays have “sides,” as Newton said, or, as explained by Dr.
Whewell, “opposite properties in opposite directions, so exactly equal
as to be capable of accurately neutralizing each other.” There are some
crystals which possess the property of “double refraction,” and thus
a ray of common light passing through such a crystal is divided into
two _polarized_ rays, taking different directions. One is refracted
according to the usual laws of refraction; the other is not, and the
planes of polarization are at right angles. It is difficult within the
limits of this chapter to explain the whole theory of Polarization. In
order to account for certain phenomena in optics, philosophers have
assumed that rays possess polarity; and polarized light is light which
has had the property of Polarization conferred upon it by reflection,
refraction, or absorption. Common light has been compared to a round
ruler, and polarized light to a flat ribbon. Huygens found out, when
engaged upon the investigation of double refraction, that the rays of
light, divided by passing through a crystal (a rhomb) of Iceland spar,
possessed certain qualities. When he passed them through a second
rhomb, he found that the brightness, relatively, of the rays depended
upon the position of the second prism, and in some positions one ray
disappeared entirely. The light had been reduced to vibrations in one
plane. In 1808, Malus, happening to direct a double refracting prism
to the windows then reflecting the sunset, found that as he turned the
prism round, the ordinary image of the window nearly disappeared in two
opposite positions; and in two other positions, at right angles, the
“extraordinary” image nearly vanished. So he found that polarization
was produced by reflection as well as by transmission. The differences
between common and polarized light have been summed up by Mr. Goddard
as follows:—
COMMON LIGHT POLARIZED LIGHT
“Is capable of reflection at “Is capable of reflection at
oblique angles of incidence in oblique angles only in
every position of the reflector. _certain positions_ of the
reflector.
“Will pass through a bundle of “Will only pass through such
plates of glass in any position glasses when they are in
in which they may be placed. _certain positions_.
“Will pass through a plate of “Will only pass in certain
tourmaline, cut parallel to the positions, and in others
axis of the crystal, in every will not pass at all.”
position of the plate.”
The bundle of glass plates or the tourmaline plate is thus the test for
polarized light, and is termed an analyzer.
The arrangement called a “Nichol’s prism,” made by cutting a prism
of Iceland spar and uniting the halves with a cement, so that only
one polarized ray can pass through it, is termed a _Polarizer_. It
only permits one of the two rays produced by “double refraction” to
pass, and the ray (as said above) will contain none but transverse
vibrations. Polarized light will produce beautiful colours. The whole
subject is very interesting to the scientist, but rather a difficult
one for the general reader to understand.
Amongst the uses to which light has been put is that of a milk-tester.
The LACTOSCOPE will show the quantity of butter contained in
a certain quantity of milk, by diluting it till it displays a certain
degree of transparency. There is another method, by the transmission of
light.
The first test is obtained by means of a glass tube about nine inches
long, closed at one end, and containing a small porcelain rod marked
with black lines. A small quantity of milk is measured and placed in
the tube. The black lines cannot at first be seen through the tube, but
by adding water the milk is rendered transparent, and the black lines
become visible. The surface of milk in the tube, by a graduated scale
upon it, shows the percentage of butter.
[Illustration: Fig. 167.—Cut card figures.]
The second method is not so simple. A short tube of tin, blackened on
the inside, and supported upright, has an opening on one side, and
opposite this, inside the tube, is a mirror placed at an angle of 45°.
“By placing a lighted candle at a known distance opposite the opening,
its light is reflected in the mirror and thrown upward through the
tube. On top of the tube is placed a round vessel of glass or metal,
closed at the bottom by a sheet of clear glass. The vessel is closed at
the top by a cover having an opening in the centre, in which slides up
and down a small tube closed at the bottom with glass, and having an
eye-piece at the top. The milk to be tested is placed in this vessel on
the top of the tin tube, so that the light of the candle reflected from
the mirror passes upward through the milk. Then, by looking through the
sliding tube and moving it up and down, a point may be found where the
image of the candle in the mirror can be seen through the milk. This
device depends, as will be seen, on observing the light transmitted
through a film of milk, and the thickness of the film is the measure of
the value of the milk. The movable tube contains a graduated scale, and
by comparison of this with a printed table, the percentage of butter in
the milk may be ascertained.”
In concluding this chapter we give a few hints for some pleasant
relaxation for young people, which has many a time created amusement.
The experiment consists in cutting out in paper or cardboard certain
portions of a face or figure, as per the illustration herewith. Fig.
167 gives the card as cut with the scissors, and the two subsequent
faces are the result of the same held at a less or greater distance
from a screen. The illustrations (fig. 168) will assist those who wish
to amuse children by making rabbits, etc., on the wall. The shadows
will be seen perfectly thrown if the hands be carefully fixed near a
good light.
[Illustration: Fig. 168.—Hand-shadows on the wall.]
We are all so familiar with the “Magic Lantern,” and the apparatus
for dissolving views by an arrangement of lenses and manipulation of
slides, that we need do no more than refer to them.
[Illustration: Fig. 169.—Dissolving views apparatus.]
The various ghost illusions, etc., produced by indirect mirrors, have
already been referred to, the ghost being merely the reflection of an
individual seen through a sheet of glass between the spectators and
the stage. The strong light throws a reflection from a parallel mirror
lower down, and this reflected image can be made to appear amongst
the real actors who are behind the plate-glass in full view of the
audience, who are, however, ignorant of the existence of the glass
screen.
For the winter evenings one may easily procure an apparatus for
dissolving views by the oxy-hydrogen light. One, as shown in the
illustration herewith (fig. 169), will answer every purpose, and by
this double arrangement phantasmagoria may be produced, or a fairy
tale may be illustrated. The effect of gradually-approaching night
may be given to the picture by means of a special glass in the lower
lanthorn. The apparatus is exhibited by means of a Drummond light, and
is very simple, although a certain supply of gas is necessary for the
performance. But this can be easily procured by an indiarubber tube,
or in a bag supplied for the purpose. Almost any objects can be used,
photographs, etc., etc., and many very comical arrangements can be made.
We have lately been reading a curious method of obtaining light
from oyster-shells in a Trans-Atlantic magazine. We give an extract
wherewith to close this chapter. The compound is “luminous paint.”
“It has been known that certain compounds of lime and sulphur had the
property of absorbing light, and giving it out again when placed in
the dark. A simple way to do this is to expose clean oyster-shells to
a red heat for half an hour. When cold, the best pieces are picked out
and packed with alternate layers of sulphur in a crucible, and exposed
to a red heat for an hour. When cold, the mass is broken up, and the
whitest pieces are placed in a clean glass bottle. On exposing the
bottle to bright sunshine during the day, it is found that at night its
contents will give out a pale light in the dark. Such a bottle filled
more than a hundred years ago still gives out light when exposed to
the sun, proving the persistency of the property of reproducing light.
Very many experiments have been more recently made in this direction,
and the light-giving property greatly enhanced. The chemicals, ground
to a flour, may now be mixed with oils or water for paints, may be
powdered on hot glass, and glass covered with a film of clear glass,
or mixed with celluloid, papier-maché, or other plastic materials. As
a paint, it may be applied to a diver’s dress, to cards, clock dials,
sign-boards, and other surfaces exposed to sunlight during the day;
the paint gives out a pale violet light at night sufficient to enable
the objects to be readily seen in the dark. If the object covered with
the prepared paint is not exposed to the sun, or if the light fades in
the dark, a short piece of magnesium wire burned before it serves to
restore the light-giving property.”