120. Furthermore, the normal eye sees an object most dis tinctly when about ten inches away. As one will seldom place a drawing nearer to his eye than the distance of distinct vision, a good general rule is to make the minimum distance between the station point and the picture plane about ten inches. For a small drawing, ten inches will be about right ; but, as the drawing in creases in size, the observer naturally holds it farther and farther from him, in order to embrace the whole without having to turn his eye too far to the right or left.
121. Sometimes a general rule is given to make the distance of the station point equal to the altitude of an equilateral tri angle, having the exfreine dimensions of the drawing for its base, and the station point for its apex.
122. The apparent distortion is always greater when the assumed position of the observer's eye is too near than when it is too far away. In the former case, objects do not seem to diminish sufficiently in size as they recede from the eye. On the other hand, if the observer's eye is between the assumed position of sta tion point and the picture plane, the effect is to make the objects diminish in size somewhat tob rapidly as they recede from the eye.
This effect is not so easily appreciated nor so disagreeable as the former. Therefore it is better to choose the position of the station point too far away, rather than too near.
123. Finally, the apparent distortion is more noticeable in curved thim in straight lines, and becomes more and more dis agreeablZ as the curve approaches the edge of the drawing. - Thus-, if curved lines occur, great pains should be taken in choosing the station point ; and, if possible, such a view of the object should be shown that the curves will fall near the center of the perspective.
124. The student should realize that the so-called distortion in a perspective projection is only an apparent condition. If the eye of the observer is placed exactly at the position assumed for it when making the drawing, the perspective projection will exactly represent to him the corresponding view of the object in space.