THE WONDERFUL EYE OF THE SUBMARINE
ONE of the gravest faults the pilots of the first submarines had to
contend with was that they were not able to see where their craft was
going.
But like everything else that is needed for the welfare or warfare
of the human race, inventors got busy and began to scheme and to
experiment, with the big idea of making an instrument which would do
for the submarine what the eye does for the brain, and that is to _look
around_ with.
=How the Eye of the Submarine Got Its Name.=—Now, the eye of the
submarine is called a _periscope_ and before we go any further let us
find out how it came to get this peculiar name for then we shall know
more of what we are talking about.
The early Greeks used two words that were very common to them, and you,
too, will use them, before you get through with this chapter, just as
easily as they did. One of these words is _peri_ which means _around_,
and the other is _scopeō_ which means to _look_; so _peri_ plus
_scopeō_ means to _around look_, or, as we barbarians say it, _look
around_—which sounds better to us. And there you have the roots of the
word _periscope_.
=The First Submarine Eye, or Periscope.=—As in the beginning of all
things, good and evil, the first attempts to make an eye for the
submarine, or a periscope, were very crude, and as usual they were
nearly, if not quite worthless.
But the periscope was no exception to the first law of invention, and
that is that each attempt, however much of a failure it may have been
in itself, had the germ of a useful device in it; and from out of all
these efforts finally came that wonderful optical instrument, the
periscope.
We say _wonderful_ because with it an observer in the conning tower of
a modern submarine, though it is all but submerged, can see everything
that is taking place on the surface of the water around the whole
horizon.
The earliest trials at making a periscope were by using a simple
arrangement of mirrors of an “L” tube and each at an angle of 45
degrees. The best way to understand its construction is to make one for
yourself.
=How to Make a Simple Periscope.=—You can get a lot of fun out of
this home-made periscope, and with it you can _out-sherlock_ Sherlock
Holmes, the great detective who was invented by Conan Doyle, for you
can see around corners, over fences, and even back of yourself just as
though you had a movable third eye, and without so much as ever being
seen yourself.
Make a tube of wood or cardboard 2 inches square and 12 inches long,
as shown in Fig. 51. Fit two square pieces of looking-glass into the
corners of the tube at 45 degrees—that is half way between the vertical
and the horizontal—and your periscope is finished and ready for use.
[Illustration: FIG. 51. HOW TO MAKE A SIMPLE MIRROR PERISCOPE.]
_How the Periscope Works._—When, now, you hold the long tube of your
periscope in a vertical position that is straight up and down, and
the light from an object, a person, or a scene at which the upper
horizontal tube, or _objective_, as it is called, is pointed, strikes
the first mirror, it bends the rays of light at an angle of 90 degrees,
when the light goes straight down the tube as shown by the arrows in
Fig. 51.
When it strikes the other and lower mirror, the rays of light are again
turned out of their path at an angle of 90 degrees, when they are
reflected out of the lower horizontal tube which forms the _eye-piece_.
If now you will place your eye to this end of the tube, you will be
able to look all around and see what you shall see. See?
=The Modern Lenticular[25] Periscope.=—The toy periscope which we have
just described, and which we hope you will make and use, does not
show what is going on at any great distance, and while this will not
interfere with your pleasure of using it, it mattered very greatly when
it was used as an eye for a submarine craft.
To be able to see farther led to the idea of using a telescope in
connection with the mirrors in the tube, and a periscope of this kind
was next made and tried out, and, let it be said, the results obtained
were a decided improvement as against those where mirrors alone were
used.
=How the Telescope Is Made.=—You may or may not know it, but an
ordinary telescope, or _spy-glass_, as it is called, is made up of four
lenses, as shown in Fig. 52.
The purpose of the large lens in the front end of the tube—or _object
glass_, to call it by its right name—is to gather in the light of the
object and form an image of it. The small lens in the back end of the
tube is used to magnify the image formed by the object glass.
Now, when a telescope has only an object-glass and an eye-piece, the
magnified image of the object looked at is always upside down; this is
the kind of telescope that astronomers use in their star work and the
fact that the image is reversed doesn’t really matter anyway because
the man in the moon looks about as well when he is standing on his head
as he does when he is right side up.
[Illustration: FIG. 52. HOW AN ORDINARY TERRESTRIAL TELESCOPE IS MADE
AND WORKS.]
But when you want to look at objects here on the earth’s surface you
want to see them as they are and not upside down. To _rectify the
image_, which means to make the eye see the object as it is, two more
lenses are placed in the tube, and so four lenses are used in all.[26]
_About the Reflecting Prisms._—The next big improvement in periscopes
came when a _total reflecting prism_ was used in the place of the
mirror at each end of the tube.
In _physics_ the terms _total reflecting_ means simply that _all_ of
the light that strikes a surface is reflected again without loss. While
a mirror will reflect only a part of the light that falls on it, a
_prism_ will reflect all of the light that enters it.
[Illustration: FIG. 53. THE REFLECTING PRISM.]
A prism is a three-sided piece of glass, if you forget to count the
ends, as shown at _A_ in Fig. 53. When a ray of light enters, say,
the vertical side of the prism it keeps on going until it strikes the
45-degree side of it; this side reflects and bends it and it passes out
of the horizontal side, as shown at _B_.
_The Construction of the Periscope._—Knowing now how a telescope is
made and what it does and also knowing what a prism is and how it
acts on light, all you have to do to understand the construction of a
submarine periscope is to take a good look at Fig. 54.
You will see that a prism is fixed in the upper end of the tube and
directly back of the object-glass; that another prism is fixed to
the lower end of the tube and back of the eye-piece; and that the
rectifying lenses of the telescope are set between these two prisms.
This picture also shows the path of the light through it.
[Illustration: FIG. 54. HOW A MODERN PERISCOPE IS MADE AND WORKS.]
These lenses and prisms are mounted in a tube about 4 inches in
diameter and 20 feet long. A horizontal revolving hood is secured to
the upper end of the tube and the horizontal eye-piece is fastened to
the lower end of it; a wheel is also fixed to the lower end of the
tube, so that the observer can turn the periscope completely around and
so scan the surface of the sea in any direction.
The periscope tube is placed in a slightly larger and very strong steel
tube, which passes through the deck of the conning tower and into the
latter, and it is made watertight by means of a _stuffing-box_.
The reason it is necessary to have a fixed outside tube is because the
force of the water, when the craft is submerged and is speeding along
under power, presses against the tube so hard that if only the inside
one were used it would bind; as it is, there is no pressure on the
inside tube and it can therefore turn freely at all times.
All recently built submarines have two periscopes, one of which leads
into the conning tower and the other one runs down into the navigating
room. Hence if one or the other is put out of commission by shell-fire,
or otherwise, the submarine can still see and find its way about.
_Gauging the Distance of an Enemy Ship._—An instrument called a
_telemeter_ (pronounced te-lem´-e-ter) is attached to the periscope
near the eye-piece, and the observer can by looking into it measure the
distance away of an enemy ship. This is done from the size of the image
it makes on the eye-piece.
Without this instrument the whole submarine, as large, as wonderful,
and as costly as it is, would be of small value, for by it the captain
is able to set his torpedo director very accurately and hence to aim
the torpedo so that it will make a sure hit.
[Illustration: _Courtesy of Scientific American_
THE LATEST TYPE OF PERISCOPE. A MUCH MAGNIFIED IMAGE OF THE OBJECT
IS SHOWN IN THE INNER CIRCLE, WHILE IN THE OUTER CIRCLE IS SHOWN THE
OBJECT PLUS AN “ALL ROUND” VIEW OF THE HORIZON. A SUBMARINE FITTED WITH
THIS PERISCOPE MAY WELL BE SAID TO HAVE EYES IN THE BACK OF ITS HEAD]
=The Latest Type of Periscope.=—As the periscope just described, and
which is still in use on submarines, has a very limited _field of
vision_ at any given setting—that is to say, only about ⅙ th of the
horizon can be seen when the instrument is pointed one way—and as a
periscope which would show the whole horizon at the same time was
badly needed, a British firm of opticians set out to invent one.
The hardest part of the task was not to get a complete view of the
whole horizon at the same time but to prevent the rays of light
which form the images from getting mixed up with one another, or
_interfering_, and so producing a blurred and indistinct picture.
The new complete-view periscope differs from the older style only in
having a circular lens and prism; these gather in the light, bend the
rays and project them on down through the telescope lens until they
reach the lower prism when all of the images are reflected into the
eye-piece where the observer sees it as a circular picture.
This new improved form of periscope is of great value, for it gives a
safety-first view on all sides of the submarine at the same time and
the observer does not need to keep turning the eye-piece, and this is
of great value when a submarine is being closed in on by two or more
enemy ships.
It is a well-known fact that a large number of submarine accidents
have been caused by the limited range of view offered by the old-style
periscope, and in some cases the undersea craft has been rammed or sunk
by gun-fire from an enemy ship which she did not see.
The new periscope makes it impossible for a destroyer to creep up
on the submarine without being caught in the act. Nearly all of the
undersea craft now being turned out by all of the warring nations are
fitted with the new 360-degree[27] vision periscope.
=The Limited Use of the Periscope.=—Notwithstanding all these latest
improvements in the periscope, its use is quite limited, for it can be
used only when the submarine is running awash or partly submerged, and
since the tube of the instrument is only 20 feet high the distance to
which a ship can be seen is about five miles.
When running partly submerged the tube of the periscope sticks up and
out of the water about 10 feet, when the distance range of vision is
then cut down to about two miles, for the curvature of the earth’s
surface meets the line of sight and everything that is at a greater
distance than this from the submarine is below the horizon and hence
invisible.
Should an enemy destroyer get within firing range of the submarine and
the captain of the latter craft wants to watch it, only the hood of the
periscope is poked up above the water; but of course the distance range
is again cut down.
The captain of a submarine has to contend with all these adverse
features of the periscope even on bright, clear days and when the sea
is calm; on dark and foggy days, when a heavy sea is running, the
periscope is next to useless, for the mist and spray gather on the
objective lenses and this makes it next to impossible to see anything.
Worst of all are the waves which break over the periscope, and this
prevents a ship from being seen even if it is only a little way off.
It is in rough weather that a submarine takes the longest chances; but
to put behind him any danger that may be lurking hard by, the captain
prefers to run undersea and come to the surface only when he has to.
A scheme to clear the moisture from the objective lenses is a device
called a _sprayer_. It is made and worked so that an observer at the
periscope can spray the lenses with alcohol. As water has a very great
liking—or _affinity_, as it is called—for alcohol, and as alcohol
evaporates almost instantly it carries the particles of water off with
it, and this helps to make the _seeing_ better.
=The New Enemy of the Submarine.=—A new enemy of the submarine has
recently made its appearance—an enemy that will make it use a periscope
of a new order.
This latest submarine destroyer is the airplane; and as the captain
cannot now see directly overhead except when his boat is running light
or awash, and the pilot of an airplane can see the submarine when it is
submerged to a very considerable depth, it is easy for him to follow
the undersea craft until she comes to the surface and then drop a bomb
on her.
A story of a running fight between an airplane and a submarine would
have put it in Col. Roosevelt’s _Ananias Club_ a few years ago, but
to-day it has all come to pass, and it looks now as if a good way
to break the backbone of Germany’s ruthless warfare on the sea is to
destroy the U-boats with a fleet of airplanes.