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Lens Story: 26 of 28

PROBABLY the most skilful adaptation of lenses and prisms to a specific purpose is to be found in the periscope. To throw horizontal rays of light down a long tube and bend them again at right angles, thereby forming an image of some distant object above, was the problem presenting itself to the inventor of the periscope.
As invented in 1854 and first used in our Civil War, the periscope consisted of a vertical tube carrying at top and bottom plane mirrors inclined at an angle of 45 degrees to the horizon. In principle it was based on the Polemoscope made by Helvelius in the seventeenth century and consisting of a telescope of the Galilean type but twice bent at right angles. Such a periscope was, however, little more than a toy. It gathered but a small quantity of light and the field of view was very limited, as was also the length of the tube. The great development of the periscope as one of the crowning triumphs of optical progress has come within the last ten years.
    The first improvement was to substitute total reflecting prisms for the mirrors but this was only a small gain. When, however, the work of Lake and Holland had demonstrated that the submarine must be seriously reckoned with as a formidable instrument of naval warfare, the necessity for a more efficient periscope became imperative. As shown in a diagram in this issue, the modern periscope preserves the total reflecting prisms at top and bottom and adds to these a system of lenses within the tube and an eyepiece below with which to observe the image. The system of lenses comprises essentially two telescopes. The upper one reduces the size of the image while the lower one magnifies it. A magnification of about one and a half times is required in order to make objects viewed through the periscope appear of normal size. The lens system is such, too, that, although the image is at first inverted, it is afterwards reinverted and therefore appears upright as viewed with the eyepiece. In many of the later types of periscopes a special erecting prism about midway of the tube serves this purpose. The rays of light from a surface object entering the window glass are reflected by the upper prism into the periscope tube, where they are successively refracted by
the upper eyepiece, upper objective, and lower objective, to the lower prism, which reflects the rays into the lower eyepiece and thence to the observer's eye.
    Periscopes are from sixteen to twenty-four feet in length. The direct vision type described above gives a range of about 60 degrees in whatever direction it may point and can be rotated through the whole circle of 360 degrees.
    It is of course highly desirable to be able to command the whole horizon in one view and for this purpose a panoramic periscope having a circular object glass has been perfected. This periscope employs a prism at the bottom of the tube and gives a circular field of view including he whole horizon but with a dark spot in the center. To fill this space a direct vision periscope is provided so that in the center is obtained an image of the scene directly ahead while about it is a fringe showing every point of the compass. The circular image always suffers distortion and, therefore, an object is simply picked up in this field but studied in detail with the direct vision periscope. Binocular eyepieces giving stereoscopic depth to periscope images have not seemed practicable and continual observation tires the eye. But in clear weather it is possible to throw these images on a ground glass screen. This is not permissible, however, when exact details are essential.
    With the periscope projecting twenty feet above the water a battleship may be picked up at a distance of about six miles in clear weather. With the periscope three feet above the water the range is restricted to about two miles and with only one foot of projecting to barely a mile. At night the periscope is of no value unless the moon is shining brightly and even then its use is very limited. Although the periscope may be shot away, water cannot enter the submarine, and, by means of compass and conning tower, escape is assured.