Now the separation which can just be distinguished is commonly assumed to be one minute of arc. This angle corresponds to about 1/3400 the distance from the eye to the object. If the object is assumed at a distance _a_ from the face, and on a line with one of the eyes, which are separated by the distance _d_, then (all angles being small) the object must be of height _a/d_ times the horizontal distance which corresponds to one minute. For 25 centimeters' viewing-distance this quantity is about 4, so that the least perceptible elevation is 4/3400 or about 1/900. The stereogram having been made under conditions giving correct relief, this fraction is also the fraction of the altitude of the plane when the photograph was taken which may be detected. An object as high as a man (6 feet) should be visible as a projection in a stereoscopic view taken at 6 × 900 = 5400 feet. This relation—1/900—holds (irrespective of the focal length of the lens), as long as the conditions for correct relief are maintained. =Stereoscopic Aerial Cameras.=—Cameras for aerial stereoscopic photography need in no way differ in construction from those made for mapping or spotting, provided only they permit exposures to be made at short enough intervals. The addition of special sights, as already discussed, constitutes the only real difference between single view and stereoscopic aerial cameras. But even without such sights ordinary aerial cameras are applicable to stereo work by the usual procedure of determining the exposure spacing by time. One scheme employed for taking low stereos, where the interval is only two or three seconds, is to mount two cameras in the plane, exposing them one after the other at the correct interval. Another method which has been tried with success is the use of a double focal-plane shutter in a single lens camera (Fig. 157). The two shutters are side by side, with their slots parallel to the line of flight. To take a stereo negative we expose first the shutter nearer the tail of the plane, and then the other, after an interval which can be calculated from the speed and altitude, or, better, determined by a stereoscopic sight. The two views are thus obtained on a single plate. Prints from these negatives are transposed right and left, and, if the prints are viewed in an ordinary stereoscope, have to be cut apart and transposed for mounting, or else this may be done to the negatives. [Illustration: FIG. 157.—Aerial hand camera fitted with two complimentary shutter slits and double sight, for stereoscopic photography.]