A submarine boat when navigating as an ordinary torpedo-boat on the surface or in the “awash” condition can be steered from a cupola or conning tower fitted with windows, which is affixed to the top of the hull, and which remains above the water when the hull is below. 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.