The advance of photography : its history and modern applications (1911)

200 THE ADVANCE OF PHOTOGRAPHY If two rectangles A and B are drawn on paper, both appear as plane figures. But directly one of them, B, is shaded with thinner or thicker lines, the rectangle no longer appears flat, but cylindrical. Thus, by imitating the gradation of light and shade, we have produced a deception for our eye. This division of light and shade is one of the most important means of producing an appearance of solidity. Perspective. — But there is another and a more important means of deception — perspective. If we observe the cube (fig. 84), the faces of which are equal, we perceive that these faces appear of very different length. The surface turned towards our eye appears a square, while the others are shortened in a marked degree, the surface appearing quite irregular, the parallel lines running together and converging to one point o, called the vanishing point. The same thing happens with all other bodies : a human arm hanging down or a standing column 8 (fig. 84) appears at their full length, but the lying column L, and the arm extended towards us, appear foreshortened. Their dimensions are contracted ; in short, we see, instead of the shaft of a column, only its circular base b, and this, again, appears sometimes round, when its full surface is turned towards us, at others an ellipse, which it is not in fact, and in this case the parallel sides of the column converge. The track of a railroad viewed in perspective presents the same features. The fact that we do not feel this deception — for such it is — to be one, results from habit. We know from experience that the arm extended towards us is longer than it appears, in perspective, to our eye, and also that the rails which appear to run together are parallel. We are continually correcting the Fig. 85.