lenses and mirror had been subjected to the most searching tests, able
to detect irregularities not exceeding 1-250,000 of an inch. M.
Gautier applied to the mirror M. Foucault’s test, which is worth
mentioning. A point of light thrown by the mirror is focused through a
telescope. The eyepiece is then moved inwards and outwards so as to
throw the point out of focus. If the point becomes a luminous circle
surrounded by concentric rings, the surface throwing the light point
is perfectly plane or smooth. If, however, a pushing-in shows a
vertical flattening of the point, and a pulling-out a horizontal
flattening, that part is concave; if the reverse happens, convexity is
the cause.
For the removal of the mirror from Jeumont to Paris a special train
was engaged, and precautions were taken rivalling those by which
travelling Royalty is guarded. The train ran at night without
stopping, and at a constant pace, so that the vibration of the glass
atoms might not vary. On arriving at Paris, the mirror was transferred
to a ponderous waggon, and escorted by a body of men to the Exposition
buildings. The huge object-lens received equally careful treatment.
The telescope was housed at the Exhibition in a long gallery pointing
due north and south, the siderostat at the north end. At the other,
the eyepiece, end, a large amphitheatre accommodated the public
assembled to watch the projection of stellar or lunar images on to a
screen thirty feet high, while a lecturer explained what was visible
from time to time. The images of the sun and moon as they appeared at
the primary focus in the eyepiece measured from twenty-one to
twenty-two inches in diameter, and the screen projections were
magnified from these about thirty times superficially.
The eyepiece section consisted of a short tube, of the same breadth as
the main tube, resting on four wheels that travelled along rails.
Special gearing moved this truck-like construction backwards and
forwards to bring a sharp focus into the eyepiece or on to a
photographic plate. Focusing was thus easy enough when once the
desired object came in view; but the observer being unable to control
the siderostat, 250 feet distant, had to telephone directions to an
assistant stationed near the mirror whenever he wished to examine an
object not in the field of vision.
By the courtesy of the proprietors of the _Strand_ _Magazine_ we are
allowed to quote M. Deloncle’s own words describing his emotions on
his first view through the giant telescope:--
“As is invariably the case, whenever an innovation that sets at nought
old-established theories is brought forward, the prophecies of failure
were many and loud, and I had more than a suspicion that my success
would cause less satisfaction to others than to myself. Better than
any one else I myself was cognisant of the unpropitious conditions in
which my instrument had to work. The proximity of the river, the dust
raised by hundreds of thousands of trampling feet, the trepidation of
the soil, the working of the machinery, the changes of temperature,
the glare from the thousands of electric lamps in close
proximity--each of these circumstances, and many others of a more
technical nature, which it would be tedious to enumerate, but which
were no less important, would have been more than sufficient to make
any astronomer despair of success even in observatories where all the
surroundings are chosen with the utmost care.
“In regions pure of calm and serene air large new instruments take
months, more often years, to regulate properly.
“In spite of everything, however, I still felt confident. Our
calculations had been gone over again and again, and I could see
nothing that in my opinion warranted the worst apprehensions of my
kind critics.
“It was with ill-restrained impatience that I waited for the first
night when the moon should show herself in a suitable position for
being observed; but the night arrived in due course.
“Everything was in readiness. The movable portion of the roof of the
building had been slid back, and the mirror of the siderostat stood
bared to the sky.
“In the dark, square chamber at the other end of the instrument, 200
feet away, into which the eyepiece of the instrument opened, I had
taken my station with two or three friends. An attendant at the
telephone stood waiting at my elbow to transmit my orders to his
colleague in charge of the levers that regulated the siderostat and
its mirror.
“The moon had risen now, and her silvery glory shone and sparkled in
the mirror.
“‘A right declension,’ I ordered.
“The telephone bell rang in reply. ‘Slowly, still slower; now to the
left--enough; again a right declension--slower; stop now--very, very
slowly.’
“On the ground-glass before our eyes the moon’s image crept up from
one corner until it had overspread the glass completely. And there we
stood in the centre of Paris, examining the surface of our satellite
with all its craters and valleys and bleak desolation.
“I had won the day.”
PHOTOGRAPHING THE INVISIBLE.
Most of us are able to recognise when we see them shadowgraphs taken
by the aid of the now famous X-rays. They generally represent some
part of the structure of men, beasts, birds, or fishes. Very dark
patches show the position of the bones, large and small; lighter
patches the more solid muscles clinging to the bony framework; and
outside these again are shadowy tracts corresponding to the thinnest
and most transparent portions of the fleshy envelope.
In an age fruitful as this in scientific marvels, it often takes some
considerable time for the public to grasp the full importance of a
fresh discovery. But when, in 1896, it was announced that Professor
Röntgen of Würzburg had actually taken photographs of the internal
organs of still living creatures, and penetrated metal and other
opaque substances with a new kind of ray, great interest was
manifested throughout the civilised world. On the one hand the “new
photography” seemed to upset popular ideas of opacity; on the other it
savoured strongly of the black art, and, by its easy excursions
through the human body, seemed likely to revolutionise medical and
surgical methods. At first many strange ideas about the X-rays got
afloat, attributing to them powers which would have surprised even
their modest discoverer. It was also thought that the records were
made in a camera after the ordinary manner of photography, but as a
matter of fact Röntgen used neither lens nor camera, the operation
being similar to that of casting a shadow on a wall by means of a
lamp. In X-radiography a specially constructed electrically-lit glass
tube takes the place of the lamp, and for the wall is substituted a
sensitised plate. The object to be radiographed is merely inserted
between them, its various parts offering varying resistance to the
rays, so that the plate is affected unequally, and after exposure may
be developed and printed from it the usual way. Photographs obtained
by using X-rays are therefore properly called shadowgraphs or
skiagraphs.
The discovery that has made Professor Röntgen famous is, like many
great discoveries, based upon the labours of other men in the same
field. Geissler, whose vacuum tubes are so well known for their
striking colour effects, had already noticed that electric discharges
sent through very much rarefied air or gases produced beautiful glows.
Sir William Crookes, following the same line of research, and reducing
with a Sprengel air-pump the internal pressure of the tubes to
1/100000 of an atmosphere, found that a luminous glow streamed from
the cathode, or negative pole, in a straight line, heating and
rendering phosphorescent anything that it met. Crookes regarded the
glow as composed of “radiant matter,” and explained its existence as
follows. The airy particles inside the tube, being few in number, are
able to move about with far greater freedom than in the tightly packed
atmosphere outside the tube. A particle, on reaching the cathode, is
repelled violently by it in a straight line, to “bombard” another
particle, the walls of the tube, or any object set up in its path, the
sudden arrest of motion being converted into light and heat.
By means of special tubes he proved that the “radiant matter” could
turn little vanes, and that the flow continued even when the terminals
of the shocking-coil were _outside_ the glass, thus meeting the
contention of Puluj that the radiant matter was nothing more than
small particles of platinum torn from the terminals. He also showed
that, when intercepted, radiant matter cast a shadow, the intercepting
object receiving the energy of the bombardment; but that when the
obstruction was removed the hitherto sheltered part of the glass wall
of the tube glowed with a brighter phosphorescence than the part which
had become “tired” by prolonged bombardment. Experiments further
revealed the fact that the shaft of “Cathode rays” could be deflected
by a magnet from their course, and that they affected an ordinary
photographic plate exposed to them.
In 1894 Lenard, a Hungarian, and pupil of the famous Hertz, fitted a
Crookes’ tube with a “window” of aluminium in its side replacing a
part of the glass, and saw that the course of the rays could be
traced through the outside air. From this it was evident that
something else than matter must be present in the shaft of energy sent
from the negative terminal of the tube, as there was no direct
communication between the interior and the exterior of the tube to
account for the external phosphorescence. Whatever was the nature of
the rays he succeeded in making them penetrate and impress themselves
on a sensitised plate enclosed in a metal box.
Then in 1896 came Röntgen’s great discovery that the rays from a
Crookes’ tube, after traversing the _glass_, could pierce opaque
matter. He covered the tube with thick cardboard, but found that it
would still cast the shadows of books, cards, wood, metals, the human
hand, &c., on to a photographic plate even at the distance of some
feet. The rays would also pass through the wood, metal, or bones in
course of time; but certain bodies, notably metals, offered a much
greater resistance than others, such as wood, leather, and paper.
Professor Röntgen crowned his efforts by showing that a skeleton could
be “shadow-graphed” while its owner was still alive.
Naturally everybody wished to know not only what the rays could do,
but what they were. Röntgen, not being able to identify them with any
known rays, took refuge in the algebraical symbol of the unknown
quantity and dubbed them X-rays. He discovered this much, however,
that they were invisible to the eye under ordinary conditions; that
they travelled in straight lines only, passing through a prism, water,
or other refracting bodies without turning aside from their path; and
that a magnet exerted no power over them. This last fact was
sufficient of itself to prevent their confusion with the radiant
matter “cathode rays” of the tube. Röntgen thought, nevertheless, that
they might be the cathode rays transmuted in some manner by their
passage through the glass, so as to resemble in their motion
sound-waves, _i.e._ moving straight forward and not swaying from side
to side in a series of zig-zags. The existence of such ether waves had
for some time before been suspected by Lord Kelvin.
Other authorities have other theories. We may mention the view that X
represents the ultra-violet rays of the spectrum, caused by vibrations
of such extreme rapidity as to be imperceptible to the human eye, just
as sounds of extremely high pitch are inaudible to the ear. This
theory is to a certain extent upheld by the behaviour of the
photographic plate, which is least affected by the colours of the
spectrum at the red end and most by those at the violet end. A
photographer is able to use red or orange light in his dark room
because his plates cannot “see” them, though he can; whereas the
reverse would be the case with X-rays. This ultra-violet theory claims
for X-rays a rate of ether vibration of trillions of waves per
second.
An alternative theory is to relegate the rays to the gap in the scale
of ether-waves between heatwaves and light-waves. But this does not
explain any more satisfactorily than the other the peculiar phenomenon
of non-refraction.
The apparatus employed in X-photography consists of a Crookes’ tube of
a special type, a powerful shocking or induction coil, a fluorescent
screen and photographic plates and appliances for developing, &c.,
besides a supply of high-pressure electricity derived from the main, a
small dynamo or batteries.
A Crookes’ tube is four to five inches in diameter, globular in its
middle portion, but tapering away towards each end. Through one
extremity is led a platinum wire, terminating in a saucer-shaped
platinum plate an inch or so across. At the focus of this, the
negative terminal, is fixed a platinum plate at an angle to the path
of the rays so as to deflect them through the side of the tube. The
positive terminal penetrates the glass at one side. The tube contains,
as we have seen, a very tiny residue of air. If this were entirely
exhausted the action of the tube would cease; so that some tubes are
so arranged that when rarefaction becomes too high the passage of an
electrical current through small bars of chemicals, whose ends project
through the sides of the tube, liberates gas from the bars in
sufficient quantity to render the tube active again.
When the Ruhmkorff induction coil is joined to the electric circuit a
series of violent discharges of great rapidity occur between the tube
terminals, resembling in their power the discharge of a Leyden jar,
though for want of a dense atmosphere the brilliant spark has been
replaced by a glow and brush-light in the tube. The coil is of large
dimensions, capable of passing a spark across an air-gap of ten to
twelve inches. It will perhaps increase the reader’s respect for
X-rays to learn that a coil of proper size contains upwards of
thirteen miles of wire; though indeed this quantity is nothing in
comparison with the 150 miles wound on the huge inductorium formerly
exhibited at the London Polytechnic.
If we were invited to an X-ray demonstration we should find the
operator and his apparatus in a darkened room. He turns on the current
and the darkness is broken by a velvety glow surrounding the negative
terminal, which gradually extends until the whole tube becomes clothed
in a green phosphorescence. A sharply-defined line athwart the tube
separates the shadowed part behind the receiving plate at the negative
focus--now intensely hot--from that on which the reflected rays fall
directly.
One of us is now invited to extend a hand close to the tube. The
operator then holds on the near side of the hand his fluorescent
screen, which is nothing more than a framework supporting a paper
smeared on one side with platino-cyanide of barium, a chemical that,
in common with several others, was discovered by Salvioni of Perugia
to be sensitive to the rays and able to make them visible to the human
eye. The value of the screen to the X-radiographer is that of the
ground-glass plate to the ordinary photographer, as it allows him to
see exactly what things are before the sensitised plate is brought
into position, and in fact largely obviates the necessity for making a
permanent record.
The screen shows clearly and in full detail all the bones of the
hand--so clearly that one is almost irresistibly drawn to peep behind
to see if a real hand is there. One of us now extends an arm and the
screen shows us the _ulna_ and the _radius_ working round each other,
now both visible, now one obscuring the other. On presenting the body
to the course of the rays a remarkable shadow is cast on to the
screen. The spinal column and the ribs; the action of the heart and
lungs are seen quite distinctly. A deep breath causes the movement of
a dark mass--the liver. There is no privacy in presence of the rays.
The enlarged heart, the diseased lung, the ulcerated liver betrays
itself at once. In a second of time the phosphorescent screen reveals
what might baulk medical examination for months.
If a photographic slide containing a dry-plate be substituted for the
focusing-screen, the rays soon penetrate any covering in which the
plate may be wrapped to protect it from ordinary light rays. The
process of taking a shadowgraph may therefore be conducted in broad
daylight, which is under certain conditions a great advantage, though
the sensitiveness of plates exposed to Röntgen rays entails special
care being taken of them when they are not in use. In the early days
of X-radiography an exposure of some minutes was necessary to secure a
negative, but now, thanks to the improvements in the tubes, a few
seconds is often sufficient.
The discovery of the X-rays is a great discovery, because it has done
much to promote the noblest possible cause, the alleviation of human
suffering. Not everybody will appreciate a more rapid mode of
telegraphy, or a new method of spinning yarn, but the dullest
intellect will give due credit to a scientific process that helps to
save life and limb. Who among us is not liable to break an arm or leg,
or suffer from internal injuries invisible to the eye? Who among us
therefore should not be thankful on reflecting that, in event of such
a mishap, the X-rays will be at hand to show just what the trouble is,
how to deal with it, and how far the healing advances day by day? The
X-ray apparatus is now as necessary for the proper equipment of a
hospital as a camera for that of a photographic studio.
It is especially welcome in the hospitals which accompany an army into
the field. Since May 1896 many a wounded soldier has had reason to
bless the patient work that led to the discovery at Würzburg. The
Greek war, the war in Cuba, the Tirah campaign, the Egyptian campaign,
and the war in South Africa, have given a quick succession of fine
opportunities for putting the new photography to the test. There is
now small excuse for the useless and agonising probings that once
added to the dangers and horrors of the military hospital. Even if the
X-ray equipment, by reason of its weight, cannot conveniently be kept
at the front of a rapidly moving army, it can be set up in the
“advanced” or “base” hospitals, whither the wounded are sent after a
first rough dressing of their injuries. The medical staff there
subject their patients to the searching rays, are able to record the
exact position of a bullet or shell-fragment, and the damage it has
done; and by promptly removing the intruder to greatly lessen its
power to harm.
The Röntgen ray has added to the surgeon’s armoury a powerful weapon.
Its possibilities are not yet fully known, but there can be no doubt
that it marks a new epoch in surgical work. And for this reason
Professor Röntgen deserves to rank with Harvey, the discoverer of the
blood’s circulation; with Jenner, the father of vaccination; and with
Sir James Young Simpson, the first doctor to use chloroform as an
anæsthetic.
PHOTOGRAPHY IN THE DARK.
Strange as it seems to take photographs with invisible rays, it is
still stranger to be able to affect sensitised plates without
apparently the presence of any kind of rays.
Professor W. J. Russell, Vice-President of the Royal Society of
London, has discovered that many substances have the power of
impressing their outlines automatically on a sensitive film, if the
substance be placed in a dark cupboard in contact with, or very close
to a dry-plate.
After some hours, or it may be days, development of the plate will
reveal a distinct impression of the body in question. Dr. Russell
experimented with wood, metal, leaves, drawings, printed matter, lace.
Zinc proved to be an unusually active agent. A plate of the metal,
highly polished and then ruled with patterns, had at the end of a few
days imparted a record of every scratch and mark to the plate. And not
only will zinc impress itself, but it affects substances which are not
themselves active, throwing shadowgraphs on to the plate. This was
demonstrated with samples of lace, laid between a plate and a small
sheet of bright zinc; also with a skeleton leaf. It is curious that
while the interposition of thin films of celluloid, gutta-percha,
vegetable parchment, and gold-beater’s skin--all inactive--between the
zinc and the plate has no obstructive effect, a plate of thin glass
counteracts the action of the zinc. Besides zinc, nickel, aluminium,
pewter, lead, and tin among the metals influence a sensitised plate.
Another totally different substance, printer’s ink, has a similar
power; or at least some printer’s ink, for Professor Russell found
that different samples varied greatly in their effects. What is
especially curious, the printed matter on _both sides_ of a piece of
newspaper appeared on the plate, and that the effect proceeded from
the ink and not from any rays passing from beyond it is proved by the
fact that the type came out _dark_ in the development, whereas if it
had been a case of shadowgraphy, the ink by intercepting rays would
have produced _white_ letters. Professor Russell has also shown that
modern writing ink is incapable of producing an impression unaided,
but that on the other hand paper written on a hundred years ago or a
printed book centuries old will, with the help of zinc, yield a
picture in which even faded and uncertain characters appear quite
distinctly. This opens the way to a practical use of the discovery, in
the deciphering of old and partly obliterated manuscripts.
A very interesting experiment may be made with that useful
possession--a five-pound note. Place the note printed side next to the
plate, and the printing appears dark; but insert the note between a
zinc sheet and the plate, its back being this time towards the
sensitised surface, and the printing appears _white_; and the zinc,
after contact with the printed side, will itself yield a picture of
the inscription as though it had absorbed some virtue from the note!
As explanation of this paradoxical dark photography--or whatever it
is--two theories may be advanced. The one--favoured by Professor
Russell--is that all “active” substances give off _vapours_ able to
act on a photographic plate. In support of this may be urged the fact
that the interposition of glass prevents the making of dark pictures.
But on the other hand it must be remembered that celluloid and
sheet-gelatine, also air-tight substances, are able to store up light
and to give it out again. It is well known among photographers that to
allow sunlight to fall on the inside of a camera is apt to have a
“fogging” effect on a plate that is exposed in the camera afterwards,
though the greatest care be taken to keep all external light from the
plate. But here the glass again presents a difficulty, for if this
were a case of reflected light, glass would evidently be _less_
obstructive than opaque vegetable parchment or gutta-percha.
SOLAR MOTORS.
One day George Stephenson and a friend stood watching a train drawn by
one of his locomotives.
“What moves that train?” asked Stephenson.
“The engine,” replied his friend.
“And what moves the engine?”
“The steam.”
“And what produces the steam?”
“Coal.”
“And what produces coal?”
This last query nonplussed his friend, and Stephenson himself replied,
“The sun.”
The “bottled sunshine” that drove the locomotive was stored up
millions of years ago in the dense forests then covering the face of
the globe. Every day vegetation was built by the sunbeams, and in the
course of ages this growth was crushed into fossil form by the
pressure of high-piled rock and débris. To-day we cast “black
diamonds” into our grates and furnaces, to call out the warmth and
power that is a legacy from a period long prior to the advent of
fire-loving man, often forgetful of its real source.
We see the influence of the sun more directly in the motions of wind
and water. Had not the sun’s action deposited snow and rain on the
uplands of the world, there would be no roaring waterfall, no rushing
torrent, no smooth-flowing stream. But for the sun heating the
atmosphere unequally, there would not be that rushing of cool air to
replace hot which we know as wind.