Vision under water is limited to but a few yards at best, and hence
a submarine boat, when submerged, would be as blind as a ship in a
dense fog and would have to grope its way along guided only by chart
and compass, were it not for a device known as a periscope, that
reaches upward and projects out of the water, enabling the steersman
to view his surroundings from the surface. Of course the height of the
periscope limits the depth at which the craft may be safely sailed. Nor
can the periscope tube be extended indefinitely, because the submarine
must be capable of diving under a vessel when occasion demands. But
when operating just under the surface, where it can see without being
seen, the craft is in far greater danger of collision than vessels
on the surface, because it must depend upon its own alertness and
agility to keep out of the way of other boats. The latter can hardly be
expected to notice the inconspicuous periscope tube projecting from the
water in time to turn their great bulks out of the danger course.
The foregoing article describes the type of periscope now in common
use on submarines and one of the engravings on this page clearly
illustrates the principles of the instrument. A serious defect of this
type of instrument is that the field of vision is too limited. The man
at the wheel is able to see under normal conditions only that which
lies immediately before the boat. It is true that he can turn the
periscope about so as to look in other directions, but this, of course,
involves considerable inconvenience. On at least two occasions has a
submarine boat been run down by a vessel coming up behind it.
[Illustration]
~SEEING IN ALL DIRECTIONS AT ONCE~
As long as the submarine has but a single eye it would seem quite
essential to make this eye all-seeing; and since the two lamentable
accidents just referred to, an inventor in England has devised a
periscope which provides a view in all directions at the same time.
This has been attempted before, but it has been found very difficult
to obtain an annular lens mirror which would project the image down
the periscope tube without distortion. The accompanying illustrations
show how this difficulty has now been overcome. While we will not
attempt to enter into a mathematical explanation of the precise form
of the mirror lens, it will suffice to state that it is an annular
prism. The prism is a zonal section of a sphere with a conoidal central
opening and a slightly concave base. All the surfaces, however, are
generated by arcs of circles owing to the mechanical inconvenience
of producing truly hyperboloidal surfaces. The lens mirror is shown
in section at _A_ in Fig. 1. The arrows indicate roughly the course
of the rays into the lens and their reflection from the surface _B_,
which is preferably silvered. The tube is provided with two objectives
_C_ and _D_ (Fig. 3) between which a condenser _E_ is interposed at
the image plane of the lens _C_. At the bottom of the periscope tube
the rays are reflected by means of a prism _F_ into the eyepiece. Two
eyepieces are employed. One of lower power, _G_, is a Kelner eyepiece,
the purpose of which is to permit inspection of the whole image, while
a high-powered eccentrically placed Huyghenian eyepiece, _H_, enables
one to inspect portions of the image. The two eyepieces are mounted in
a rectilinear chamber, _I_, which may be rotated about the prism at
the end of the periscope, thus bringing one or other of the eyepieces
into active position. The plan view, Fig. 4, shows in full lines the
high-powered eyepiece in operative position, while the dotted lines
indicate the parts moved about to bring the low-powered eyepiece into
use. A small catch, _J_, shown in Fig. 2, serves to hold the chamber in
either of these two positions. The high-powered eyepiece is mounted on
a plate, _K_, which may be rotated to bring the eyepiece into position
for inspecting any desired portions of the annular image. The parts
are so arranged that when the eyepiece is in its uppermost position,
as indicated by full lines in Fig. 2, the observer can see that which
is directly in front of the submarine, and when the eyepiece is in its
low position, as indicated by dotted lines, he sees objects to the
rear of the submarine. With the eyepiece at the right or at the left
he sees objects at the right or left, respectively, of the submarine.
The high-powered eyepiece is slightly inclined, so that the image may
be viewed normally and to equal advantage in all parts. Mounted above
a plain unsilvered portion of the mirror is a scale of degrees which
appears just outside of the annular image. A scale is also engraved on
the plate _K_ with a fixed pointer on the chamber, making it possible
to locate the position of any object and rotate the plate _K_ so as
to bring the eyepiece _H_ on it. The scale also makes it possible to
locate the object with respect to the boat.
[Illustration: HOW WE LOOK THROUGH A PERISCOPE