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

ART IN PHOTOGRAPHY 201 errors of our sense of sight. The eye gives us a false representation of objects, and the painter takes advantage of this circumstance. He represents the lying column L b, and the sides of the cube, as falsely as we see them — that is, " foreshortened " in their dimensions, with their parallel lines converging — and everyone is deceived by this. It is the task of the artist as of the photographer to represent perspective correctly — that is, as it appears to our eye. If this is not the case, the picture appears incorrect. Perspective teaches us the laws of foreshortening. Our eye is a camera obscura with a simple landscape lens. It is known from optics that the image of A any point lies on the / straight line drawn from d£^~ the point to the optical t centre of the objective, at the place where this line, named the principal radius, cuts the plane of the image — the ground-glass screen of the camera or the retina of our eye. The image of a straight line is the place where the rays from each point of the line, passing through the optical centre, cut the groundglass screen. Now, these rays form a plane, and this plane cuts the flat screen in a straight line. Therefore the image of a straight line is to our eyes another straight line, and the image of a plane triangle another plane triangle. If the flat figure is parallel to the retina, by well-known stereometric laws the image is like the original. Let the reader imagine a glass slab placed perpendicular to the axis of his eye ; then the rays or pencils of light issuing from this object abed will cut it so as to form a figure a' b' c' d' (fig. 86). If such a figure is drawn for a given point of intersection, this drawing, if brought to a proper position