I will briefly enumerate a few of the more important of these devices and describe their function as applied to the war submarine. [Illustration: THE LOWER PORTION OF GALILEO PERISCOPE] [Illustration: THE PERISCOPE IS THE EYE OF THE SUBMARINE. (See description.)] =The Periscope.=--The periscope is the eye of the submarine. In its simpler form it consists of a stiff metallic tube, from fifteen to twenty feet in length and about four inches in diameter. Referring to Figure 1, on page 23, it is made up of an object glass, _A_, which "views" or takes an impression of all objects within its range or field of vision, and transmits an image of such object through the right-angle prism, _B_, which turns the image so that it appears some distance down the tube, say, for purposes of description, at _C_. If a piece of ground glass were held at the focus of the objective lens at _C_, the image could be seen. The lens _D_, located farther down the tube, in turn now "views" the image and transmits it still farther down the tube, where it is turned through the right-angle prism, _E_, and where the image is again turned into an erect position. A piece of ground glass located at _F_ would show the image in the same manner as an image is shown on the ground glass of a camera. The magnifying eyepiece _G_ magnifies the image so that distant objects appear of natural size. Other figures show a periscope as made by the Officina Galileo in Florence, Italy. This firm makes periscopes with binocular eyepieces. The success of any periscope depends upon the character of the material used in the lenses and prisms and the accuracy of the workmanship. This firm, which is probably the oldest optical manufacturing house in the world, said to have been founded by Galileo himself, turns out instruments of the most beautiful workmanship. The flange of the instrument is bolted to the top of the conning tower, or deck, and a gate valve is arranged between the deck and the eyepiece so that in case the tube should be carried away the gate valve can be closed and thus prevent water from entering the vessel. A hand wheel arranged below the binocular eyepiece permits of easy rotation of the instrument. Provision is made for introducing dry air; this prevents condensation forming on the lenses or prisms within the tube. Owing to the fact that there is a certain loss of light in transmitting the image through the various prisms and lenses, it is customary to magnify the image so that it appears to be about one-quarter larger than when viewed by the natural eye. This has been found by experience to give, when viewed through the periscope alone from a submerged vessel, the impression of correct distance.