National Geographic : 1918 Jan
© Paul Thompson AN AIRPLANE OBSERVER WHO COULD NOT PASS THE STEREOSCOPIC VISION TEST WOULD BE A LIABILITY It is essential for the airman to be able to judge correctly whether one object is nearer than another; whether one machine is higher than another; whether one battery is further advanced than another. Other wise he could not direct artillery fire accurately, nor could he protect himself against the attack of enemy aviators. ©Paul Thompson THE MAN WHO SEESDOUBLE ISNOT DESTINED TOBECOME AN AIR PILOT In the muscle balance test thecandidate watches acandle flame through a peculiar contrivance. The examiner gradually adjusts the apparatus, causing the flame toappear double, one above theother. A spirit-level indicates thepoint atwhich theflame should appear dupli cated. This is one of themost important tests.