This new improved form of periscope is of great value, for it gives a safety-first view on all sides of the submarine at the same time and the observer does not need to keep turning the eye-piece, and this is of great value when a submarine is being closed in on by two or more enemy ships.
It is a well-known fact that a large number of submarine accidents have been caused by the limited range of view offered by the old-style periscope, and in some cases the undersea craft has been rammed or sunk by gun-fire from an enemy ship which she did not see.
The new periscope makes it impossible for a destroyer to creep up on the submarine without being caught in the act. Nearly all of the undersea craft now being turned out by all of the warring nations are fitted with the new 360-degree[27] vision periscope.
The Limited Use of the Periscope.—Notwithstanding all these latest improvements in the periscope, its use is quite limited, for it can be used only when the submarine is running awash or partly submerged, and since the tube of the instrument is only 20 feet high the distance to which a ship can be seen is about five miles.
When running partly submerged the tube of the periscope sticks up and out of the water about 10 feet, when the distance range of vision is then cut down to about two miles, for the curvature of the earth’s surface meets the line of sight and everything that is at a greater distance than this from the submarine is below the horizon and hence invisible.
Should an enemy destroyer get within firing range of the submarine and the captain of the latter craft wants to watch it, only the hood of the periscope is poked up above the water; but of course the distance range is again cut down.
The captain of a submarine has to contend with all these adverse features of the periscope even on bright, clear days and when the sea is calm; on dark and foggy days, when a heavy sea is running, the periscope is next to useless, for the mist and spray gather on the objective lenses and this makes it next to impossible to see anything.
Worst of all are the waves which break over the periscope, and this prevents a ship from being seen even if it is only a little way off. It is in rough weather that a submarine takes the longest chances; but to put behind him any danger that may be lurking hard by, the captain prefers to run undersea and come to the surface only when he has to.
A scheme to clear the moisture from the objective lenses is a device called a sprayer. It is made and worked so that an observer at the periscope can spray the lenses with alcohol. As water has a very great liking—or affinity, as it is called—for alcohol, and as alcohol evaporates almost instantly it carries the particles of water off with it, and this helps to make the seeing better.