Can the actual difference in elevation be discovered by the use of stereoscopic views? An approximate idea may be obtained from the following considerations: Suppose we have two small point-like objects, one above the other, such as a street lamp globe and the base of the lamp pillar. In a view taken from directly overhead these will be superposed, and so will not be capable of separation. But, as the point of view is shifted sideways, the two objects separate, until a point is reached where they can just be distinguished as double. When this condition holds for either picture of the stereoscopic pair it will be possible to obtain stereoscopic relief.

Now the separation which can just be distinguished is commonly assumed to be one minute of arc. This angle corresponds to about 3x00 tne 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 -j times the horizontal distance which corresponds to one minute. For 25 centimeters' view-ing-distance this quantity is about 4, so that the least perhaving 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 6X900 = 5400 feet. This relation— q^-q —holds (irrespective of the focal length of the lens), as long as the conditions for correct relief are maintained.