It is based upon a peculiar property of a triangle. In the case of every triangle which has straight sides, if we know the size of two of the angles and the length of one of the sides we can easily calculate all that there is to be known about that triangle. We unconsciously use the principle when we judge a distance with our eyes. We focus each eye separately upon the object which we are looking at. In other words, each of our eyes looks along a straight line terminating in the object. Those two lines, together with a line joining our two eyes, form a triangle. The line between our eyes is the "base," the line of which we know the length, while the directions in which we point our eyes give us the angles at each end of the base. From this we are able to judge the distance of the object. Of course there is probably not one of us who knows the length of that natural "base" in inches, but that does not matter in this case, since it is always the same whatever we may look at, and so the mere inclination of the eyes gives us a means of comparing distances. When we judge by the eye alone, what we really do is to draw upon our experience and consciously or unconsciously compare the distance which we are estimating with some others which we already know. In surveying, a telescope is set up at one end of a base-line and pointed first at the other end of the base-line and then at the distant object. A scale with which the instrument is provided gives us the size of the angle between the two. Then the same thing is done at the other end of the "base" and the similar angle there is obtained. The length of the base being known, the distance of the remote object can then be calculated. In the same way two observations can be made, one at each end of a ship, the length of the ship forming the base-line. Or an instrument can be made by which two observations can be made simultaneously by the same man. This is done by means of mirrors which are turned so that the same object is seen in both of them, apparently in a straight line. The extent to which one of them has to be turned gives the angle, and the instrument forms the base. Anyone with the slightest geometrical experience will perceive at once that the best results are obtained when the base-line is of considerable length, and hence small portable range-finding instruments such as can be easily carried and used by one man are necessarily less accurate than an arrangement such as has been suggested above, where two observers work simultaneously from the two ends of a ship. In many cases, however, the self-contained instrument is the only one which it is possible to use, and when the instrument is well made and in experienced hands the results are surprisingly good.