Abstract
THE controversy in NATURE on this subject has brought back to my thoughts a singular illustration of the power of trained eyesight which seems worth noting, though it does not touch the exact comparison between savage and civilised eyes which is the immediate subject of the letters which have appeared in your columns. I refer to the vastly greater capacity for determining visual direction supplied by the sense of symmetry than by actual discrimination between two slightly distant visible points. If you look at a circle, you can aim at its centre with far greater exactitude than you could aim at a point in the true centre of the figure. Every rifleman and every billiard-player exemplifies this. Suppose a billiard-ball placed a little less than five feet from a pocket, and played at as a half-ball stroke from an equal distance for a winning hazard. This is something like what has to be done from baulk in making a pair-of-breeches stroke into the corner pocket, A fair amateur will pot his ball pretty often; a first-rate professional will do it very often. No one, perhaps, can make it a really safe stroke. But observe the accuracy required. The margin of error allowed on each side of the perfect stroke is, on a severe table, not more than an inch at the pocket. This allows an error on each side of about one degree in the point of impact with a radius of one inch (the ball being two inches in diameter). This one inch subtends at the distance from which the stroke is played (nearly 5 feet), an angle of 1° × sin 60°, 1/60 = about .8′. To make the stroke you must first, by eye, place your striking-ball right, then you must, by eye, aim the stroke right, and finally you must make the muscles follow the eye rightly. These three elements of error combined must leave a resultant error of not more than four-fifths of a minute; that is to say, a successful stroke must have a total angular error very considerably less than the smallest angular distance which the eye can appreciate between two visible points. This, of course, explains also the superiority of a rifle foresight, which surrounds the object by a symmetrical figure over one which depends on making one point visibly cover another.
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H., G. Civilisation and Eyesight. Nature 31, 408 (1885). https://doi.org/10.1038/031408a0
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DOI: https://doi.org/10.1038/031408a0
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