I have drawn a sketch--in which the scale of distance is exaggerated in order to better illustrate my meaning--of the earth's surface to show the comparative visibility of vessels when seen from a military submarine, lying in wait, with periscope extended fifteen feet above water. Now take such ships as the _Lusitania_, shown in position No. 5 on the diagram, with her smoke-stacks extending over one hundred feet above the surface of the sea; their tops would appear above the horizon and become visible to a distant observer with a powerful glass, stationed at, say, fifteen feet above the surface, at a distance of about eighteen and three-eighths miles. Her smoke-stack would also become visible through a telescopic periscope, the object glass of which was extended fifteen feet above the surface, while men seated in a rowboat could not see each other because of the intervening "hill," so to speak, at a distance of four miles apart. If they were under water in a submarine they could not see each other at all unless they had the periscopes elevated above the surface. In that case it would not be possible for one periscope to see another at any considerable distance, because the periscope is such a small object, and vision through it does not compare with natural vision, owing to the fact that there is considerable loss of light in passing the image of external objects through lenses and prisms. Hence it has been found necessary to reduce the field of vision to about one-half that of natural vision to give the effect of true distance, and as soon as twilight falls it is practically useless. I have taken fifteen feet above the surface without the submarine's conning tower showing, for if her conning tower is shown above the surface she is in danger of being herself discovered.