114. •There seems to exist in the minds of some beginners in the study of perspectiVe, the idea that the drawing of an object made in accordance with geometrical rules may differ essentially from the appearance of the object in nature. • Such an idea is erroneous, however. The only difference between the appearance of a view in nature and its correctly constructed perspective pro jection is that the view in nature may be looked at from any point, while its perspective representation shows the view as seen from one particular point, and from that point only.
• For every new position that the observer takes, he will see a new view of the object in space, his eye always being at the apex of the cone of visual rays that projects the view he sees (see Fig. 1). In looking at an object in space, the observer may change his position as often as he likes, and will see a new view of the object for every new position that he takes., 115. This is not true of the perspective projection of the object, however. Before making a perspective drawing, the posi tion of the observer's eye, or station point, must be decided upon, and the resulting perspective projection will represent the object as seen from this point, and from this point only. The observer, when looking at the drawing, in order that it may correctly repre sent to him the object in space, must place his eye exactly at the assumed position of the station point. If the eye is not placed exactly at the station point, the drawing will not appear abso lutely correct, and under some conditions will appear much dis torted or exaggerated.
116. Just lrere lies the defect in the science of perspective. It is the assumption that the observer has but one eye. Practically, of course, this is seldom the case. A' drawing is generally seen with two eyes, and the casual observer never thinks of placing his eye in the proper position. Even were he inclined to do so, it would generally be beyond his power, as the position of the station point is seldom shown on drawing.
117. As an illustration of apparent distortion, consider the perspective projection shown. in Fig. 23. In order that the per spectives of the vanishing points might fall within the rather narrow limits of the plate, the station point in the figure has been assumed very close to the picture plane, the distance from IIPP to SP" showing the assumed distance from the paper at which the observer should place his eye in order to obtain a connect view of the perspective projection. This distance is so short it is most improbable that the observer will ordinarily place his eye in the proper position when viewing the drawing. Consequently the perspective projection appears more or less unnatural, or dis torted. But, for the sake of experiment, if the student will cut a small, round hole, one quarter of an inch in diameter, from a piece of cardboard, place it directly in front of SPY and at a dis tance from the paper equal to the distance of SP" from IIPP, and if he will then look at the drawing through the hole in the cald board, closing the eye he is not using, he will find that the unpleas ant appearance of . the perspective projection disappears.
It will thus be seen that unless the observer's eye is in the proper position while viewing a drawing, the perspective 'projec tion may give a very unsatisfactory representation of the object in space.
118. If the observer's eye is not very far removed from the correct positio'n, the apparent distortion will not be great, and in the majority of cases will be unnoticeable. In assuming the posi tion for the station point, care should be taken to choose such a position that the observer will naturally place his eyes there when viewing the drawing.
119. As a person naturally holds any object at which is looking directly in front of his eyes, the first thought in assuming the station point should be to place it so that it will come very nearly in the center of the perspective projection.