The Submarine Torpedo Boat
Its Characteristics And Modern Development

94 THE SUBMARINE TORPEDO BOAT gether very unsatisfactory. The modern periscopes are constructed so that they may be revolved in any direction and the image remain always erect, and in addition they are fitted with a movable pointer on a fixed dial which indicates the bearing of the object with respect to the axis of the boat. Ordinarily the periscopes are used without the aid of magnifying glasses in order that the image may appear at its true distance, but monocular and binocular magnifying eyepieces are fitted so as to be quickly brought into operation that the object may be picked up and made clearly distinguishable. Various means for range finding have been fitted to the periscopes. One method is to project telemeter scales or cross-hairs into the eyepiece of the instrument, grad- uated vertically and horizontally in hundredths. If an object of known dimension under observation measured five gradations in the eyepiece its distance would be one hundred times the corresponding real dimension divided by five. Several variations of this system are in use, but they are all open to the same objection, that the dimensions of the object must be assumed and that even if a good guess is made the object may present a projected view to the observer instead of a broadside view and this consid- erably shortens the true length. Another method of which there are also variations is the double-picture mi- crometer arrangement, in which two pictures of the same object cut each other in the lens. The two pictures may be shifted with reference to one another, until the tops of the masts of a ship under observation in one picture are level with the water line in the other, and the angle of shift measured to determine the distance. This method too, is only roughly approximate for it has to deal with