Estimation by Eye

Estimation by eye, the ability to determine distances visually, without producing direct measurements of this distance. Estimation by eye has great significance in daily life, when a person needs to distinguish objects lying closer and farther away, and plays an enormous role in so-called estimation-by-eye surveys. When determining distance by estimation by eye, direct the eye to the point in space being studied and evaluate using muscular sensation those muscles which produce the movement of the eye, the tension necessary to turn the eye and direct the line of sight to this point in space. Thus, for example, if we have points lying in the horizontal plane along a line situated in the sagittal plane, then the convergence of the two eyes will be greater the closer the distance is from us to the point being studied. Thus, for points lying at infinity, the lines of sight are parallel, while for points lying at close distances, the convergence of the lines of sight reaches a significant magnitude. Determination of distances by estimation by eye is more precise the closer the object being studied is to the eye. For objects lying very far away, estimation-by-eye determination becomes very inaccurate, and there exists a whole series of methods that allow refining these determination techniques. (A more detailed exposition of the principles related to estimation-by-eye determination of distances—see Binocular Vision, Stereoscope.) It is only necessary to indicate that, by using a special instrument, it is possible to increase the baseline distance at which estimation-by-eye determinations are made—one can as if spread apart the centers of rotation of our eyes and thus obtain the possibility of determining by estimation by eye such distances that are inaccessible for determination under ordinary conditions. Estimation by eye in military affairs. Estimation by eye has great practical significance in military affairs. If in some types of weapons correct estimation by eye ensures the successful completion of a task (marksmen, observers), then in others, for example for a pilot, an error in determining distance by estimation by eye can lead to an accident. The necessity of determining absolute distance by estimation by eye is encountered: in infantry—when firing from a rifle, in cavalry—when taking a barrier, in artillery—when adjusting fire, in conditions of flight service—during air combat, when landing an airplane in winter on a snow-covered expanse of earth, a seaplane on a mirror-like water surface, during figure flying, when maintaining intervals in group flights, etc.; with determination of relative distance by estimation by eye—during normal landing on an area covered with grass, when taking off an airplane, etc. The greatest difficulty is presented by the determination of absolute distance. For developing estimation by eye in troops, training methods are applied, based on empirical data, such as the visibility of objects depending on the distance of observation. Noishchevsky proposes the following formula, based on the same principle of visibility: D (distance) = f (thickness of objects) × 3,000. The method has a number of shortcomings. The same author for practical exercise in determining distance by estimation by eye proposes to use the "Table for Research of Visual Acuity with the Help of Letters and Squares," proceeding from the position that a line 1 mm thick a normal eye distinguishes at a distance of 3 m, and 1 m thick—at a distance of 3 km. Observations show that for correct estimation by eye, skill is necessary. In the RKKA with the development of scientific research laboratories, attention was paid to developing a method for researching estimation by eye as an individual ability. The following methods are used: the classic method of Hering with a fraction, the Howard-Dolman method (for determining relative distance by estimation by eye), stereoscopes (see), Lehmann's instrument, the estimation-by-eye ruler (for determining the ability to evaluate linear dimensions), and some others.