From the beginning of the 20th century, however, the practical introduction of submarine navigation brought about the development of new elaborate periscopes of great length and provided with an optical system of lenses, which were built into the structure of the submarine.
In the present article, periscopes for land service and those forming part of the equipment of submarines will be described in turn.
(i) Land-service Periscopes vary much both in design and size, some being only a few inches long while others are as much as 80 ft.
Less conspicuous periscopes were therefore designed, and these, in order to take in enough of the foreground, had to be provided with a magnifying as well as a reflecting system.
Small German periscopes were usually 1 metre or 2 a metre in length and had two eyepieces giving magnifications 10 and 15 diameters.
Many periscopes of considerable length and special design have been used, to enable observations being made in comparative safety from behind large objects, e.g.
To enable a distant ship or other object to be examined more closely it is possible in some periscopes to change the magnification from a normal power of 1.5 to a power of 6.
As a rule every submarine has at least two periscopes, one unifocal with a small upper tube and the other bifocal and sky-searching with a larger upper tube.
Whilst in the British service sky searching up to right overhead was arranged for, German periscopes as a whole are limited to 20° above the horizontal.
Although two periscopes are provided when attacking, one only would be shown for short periods to get check observation so as to prevent the wash of the upper tube revealing the proximity of the submarine.