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.
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
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.
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.