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.
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.
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.