Having established the correct relation of taking points for stereos the next problem is how to determine these when in the plane. The simplest way is by means of a _stereoscopic sight_. This consists essentially of two lines of sight (fixed by beads, crosses, or other objects), inclined toward each other at the angle determined by the ratio of the ocular separation to the focal length of the lens. If the back sight is made a single bead or cross, the rest of the stereo sight will consist of two beads or crosses, separated from each other by the ocular distance of 65 millimeters, and distant from the back sight by the focal length of the lens (Fig. 157). The first picture is taken when the object is in line with the forward pointing line of sight, the second when it lies along the backward pointing one. Like other sights, the stereoscopic sight may be attached either to the camera, or if this is fixed in position, to any convenient part of the plane. A very simple sight for vertical stereoscopic photography consists of an inverted V painted on the side of the fuselage, so that the eye can be placed at the vertex and sighted along either leg. The common method of determining the space between exposures is by the _time_ interval. If _V_ is the speed of the plane, and _t_ the desired time interval, we have, from the last equation— _D_ _dA_ _t_ = ——— = ———— _V_ _aV_ If _A_ = 2000 meters, _d_ = 65 millimeters, and _a_ = 25 centimeters, and if the plane is traveling 200 kilometers per hour, the time interval must be— .065 × 2000 × 3600 —————————————————— = 9.4 seconds .25 × 200,000 At 1000 meters altitude the interval will be half this, and so on in proportion. If the pictures are taken with a 50 centimeter focus camera, and are hence to be viewed at 50 centimeters convergence distance instead of at 25, the time will again be halved. These relations are clearly shown in the diagram (Fig. 156). Here the left-hand portion shows how to find the stereoscopic base line at each altitude for each focal length; while the right-hand portion shows how to translate this into time interval for any plane velocity. The Burchall slide rule (Fig. 130) shows another way to arrange these data in form for rapid calculation. [Illustration: FIG. 156.—Chart for calculating intervals between exposures for stereoscopic pictures.]