It was asserted that such a cupola would not only be of no use beneath the waves, but would also be a disturbing element in the equilibrium of the boat, reducing its speed. For this reason the Gymnote and the Gustave Zédé were provided with telescopic domes capable of being pushed up or down at will. The arrangement was very complicated, and did not give good results, so it was abandoned, and all modern submarines carry a fixed dome on their deck platform.
When the submarine is submerged to a depth not greater than 10 to 12 feet such aids to vision as the optical tube and the periscope may be employed, but when the depth exceeds this limit the helmsman must rely on his compass, coming from time to time to the surface to verify, and, if necessary, rectify his route.
The periscope (from the Greek περι, around, and σκοπειν, look) is applied to an instrument by which objects in a horizontal view may be seen through a vertical tube. It may be said to consist of a vertical tube with a lenticular total-reflection prism at the top by which horizontal rays are projected downward through the tube and brought to a focus, after which they are received by a lens, the principal focus of which coincides with that point. The vertical cylindrical beam thus formed is converted into a horizontal one again by a mirror inclined at 45° from the vertical axis of the tube, and is thus conveyed to an eyepiece through which, by turning the tube on its vertical axis with its attached prism, a view of all the supernatant objects around the vessel may be obtained. A screen or diaphragm operated by a tangent screw is used to cut off the view of the vertical plane in which the sun is. When used on a submarine boat the top of the periscope floats on the surface.
The optical tube with which the Gustave Zédé was at first provided before she carried a periscope consisted of a lens and a prism on the top of a tube, and the image of the surface was thrown on to a surface of paper. By the aid of the picture on the sheet of white paper the steersman could, under certain circumstances, tell approximately where he was going.
A writer in a recent number of the Debats has something to say about vision below water.
“We certainly possess these vessels capable of navigating under water, which is no small advantage, but in order that they may be able to fight under these conditions and become really effective fighting machines the question of vision will have to be perfectly solved. The Moniteur de la Marine affirms that the periscope, the apparatus intended to secure this power of vision, perfectly fulfils its object. It gives, six metres under water, a good enough view of what is going on upon the surface to enable a boat to steer towards the enemy’s vessel without the necessity of rising to the surface to make observations which quick-firing guns would render dangerous. No less an assertion than this is necessary to shake certain doubts on this point which will be shared by all who have been in a position to observe the troubles caused to the vision by the water and vapour on ground glasses. Only those who have used this instrument have the right to say with Polycente, ‘Je vois, je sais, je crois, je suis illuminé.’
“We who have not seen cannot go so far as this. It is also necessary that the images given by the periscope should be susceptible of rapid measurement, sufficiently accurate for the officer in command to be able to know if he is near enough for his torpedo to prove effective.
“In a word, the Sous-Marin and the Submersible are at present marvels of applied mechanics. They will be fighting machines only when the problems of vision and accurate measurement of distances are fully solved. If this result is reached we shall not long be kept in the dark, for with modern parliamentary methods all over the world we shall witness the simultaneous appearance in all naval budgets of demands for considerable sums in order to construct these vessels.”
A device for submarine vision, termed the cleptoscope, and invented by Messrs. Russo and Laurenti, is reported to be used on the Italian submarine Delfino. In its original form the instrument, according to the Lega Navale, gave an exact view in a closed chamber of all that was to be seen round about a submarine to any one applying his eye to a small eyepiece. In its later form it gives the same image much enlarged, and visible to both eyes at once at some distance from the chamber.
As an illustration of the disadvantages arising from the fact that the submarine is blind, the following story may be told, the accuracy of which cannot however be vouched for.