he object picked up by the periscope became inverted.
Their radius of vision was limited, and in every way they proved
unreliable and unsatisfactory. But, just as almost every feature of
submarine construction was gradually developed and most every
technical obstacle overcome, experts gradually concentrated their
efforts on the improvement of periscopes. Modern periscopes are
complicated optical instruments which have been developed to a very
high point of efficiency. A combination of prisms and lenses makes
it possible now to see true images clearly. Appliances have been
developed to make the rotation of the periscope safe, prompt, and
easy so that the horizon can be swept readily in every direction.
Magnification can be established at will by special devices easily
connected or disconnected with the regular instrument. The range of
vision of the modern periscope is as remarkable as its other
characteristics. It differs, of course, in proportion to the height
to which the periscope is elevated above the surface of the water.
In clear weather a submarine, having elevated its periscope to a
height of 20 feet can pick up a large battleship at as great a
distance as 6 miles, while observers on the latter, even if equipped
with the most powerful optical instruments, are absolutely unable to
detect the submarine. This great distance is reduced to about 4000
yards if the periscope is only 3 feet above the surface of the water
and to about 2200 yards if the elevation of the periscope is 1 foot.
But even the highly developed periscope of to-day, usually called
"panoramic periscope," has its limitations. The strain on the
observer's eyes is very severe and can be borne only for short
periods. In dirty weather the objectives become cloudy and the
images are rendered obscure and indefinite, although this trouble
has been corrected, at least in part, by forcing a strong blast
through the rim surrounding the observation glass. At night, of
course, the periscope is practically useless. Formerly a shot which
cut off the periscope near the water's edge might sink the boat.
This has been guarded against by cutting off the tube with a heavy
plate of transparent glass which does not obstruct vision but shuts
off the entrance of water.

Important as the periscope is both as a means of observing the
surroundings of the submarine and as a guide in steering it, it is
not the only means of accomplishing the latter purpose. To-day every
subma