CERTAIN rare individuals are able to read letters only 10 millimeters high - about one-third of an inch - from 20 feet away, but their eyes are far sharper than Uncle Sam needs in his warplanes. Applicants for service in the U.S. Army Air Forces are required to read letters twice that size, 20 millimeters high, from the same distance. Twenty millimeters is slightly more than three-fourths of an inch. Try it yourself. “Just normal visual acuity,” flight surgeons of the School of Aviation Medicine call it. That word normal describes exactly the requirements for aviation cadets - normal eyes, normal heart, normal reactions, normal muscular balance. Instead of being so rigid that only trained athletes - “supermen” - can pass them, the tests are not too difficult for any normal college man. Among the aviation cadet’s tests at Randolph Field, Texas, is that of depth perception, the ability to judge comparative distances of objects. Two black rods are mounted on a sliding base. Strings are attached to these rods so that the pilot-to-be can move them from a distance. The idea is to adjust the rods so that they will be more or less equidistant from your eyes. An error of 30 millimeters is allowed. “Just normal,” examining doctors say, for some persons can adjust the sticks with an error as small as five millimeters. Can you make your hands and feet behave? Well, the aviation cadet must, to a certain extent. Into the chair of the automatic serial reaction machine he goes. Facing him are double rows of lights. One row is red and the other is green. Also in front of the applicant is a control stick and rudder pedals, like those in an airplane. Instead of controlling a plane, they control the lights. The entire machine is automatic from this point on. Three red lights flash on, and the applicant moves the controls until the green lights are lighted directly opposite the red ones. When correct, the time to perform this task is registered. Another set of red lights flashes on, then another and another until the applicant has solved 40 settings. This determines his reaction time. From there the cadet-to-be may go to the Barany chair, in which he is spun round and round to test the reactions of the middle ear, where the sense of balance is located. One forbidding-looking instrument is the phorometer, which checks for sharpness of vision and eye-muscle balance. Even the stereopticon, a familiar fixture in every parlor two generations ago, has its place among the test equipment. Here it is known as the Keystone stereoscope and it is used for research in visual acuity, fusion testing and depth perception tests. Over in another corner is a perimeter, an instrument that determines whether the applicant’s indirect vision - from out of the “corner” of his eye—is normal. Still another eye test is that for color blindness. If the applicant has trouble in distinguishing the “stop” light from the “go” light, he probably will have difficulty at this point. Another odd instrument looks like a homemade telescope. It tests the visual acuity - sharpness of vision - of the applicant with a low degree of illumination. Stepping up and down on a two-foot stool doesn't look like violent exercise, but it changes blood pressure and pulse rate sufficiently to give the flight surgeons a good indication of the condition of the heart. Blood pressure reclining, pulse rate reclining both are checked against the same readings taken while standing up and also after exercise. A numerical value is given each reading and all combine to give a final result. An 18 is perfect, the doctors say, but potential pilots need only a plus eight. Research goes on constantly at the School of Aviation Medicine in order that the flight surgeons may determine more accurately the precise requirements for all types of fighting pilots. For instance, medical officers at Randolph Field subject themselves to practical tests to learn more about oxygen starvation as the result of high altitude flying. So they climb into a plane and don oxygen masks as the ship soars into rarefied air. Blood pressures and the ability to solve arithmetic problems quickly are tested. The results are recorded electrically - and they may come in handy if any pilot ever is forced to fight 25,000 or 30,000 feet above the earth. In another direction, the medical officers study electrically recorded brain waves in an effort to determine what it takes to make a military aviator. But all in all, a “good little man” and a “good big man” are all equal when it comes to flying an airplane, so Uncle Sam’s examiners simply look for the “normal” man, not the superman.