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

202 THE ADVANCE OF PHOTOGRAPHY If and distance from the eye, will produce on it exactly the same impression as the object itself. This is the secret of the solid appearance of plane pictures properly constructed. A picture made in the manner just described is named a drawing in perspective. It is evident that such a drawing must be viewed under the same conditions as those in which it was designed. If A B G D (fig. 87) is the outline of a house, B the picture plane, 0 the point of intersection of the rays, and abed the image of the points A B C D, the eye must be 0 brought exactly to the point of intersection 0 if the representation in perspective a b c d is to produce the same impression as the object, brought nearer to the eye (for example, to B'), it is evident that the rays will intersect at a very different angle from those issuing from the object A B C D ; accordingly, they cannot produce a correct impression. The same thing would be the case if the picture were removed further from the eye {e.g. to B"). Therefore every drawing in perspective must be viewed from the point of intersection of the rays adopted as the basis of its construction if it is to produce a correct impression. Now, photography is a drawing in perspective whose point of sight is in the objective. Accordingly the inspecting eye must be brought to the same distance as the objective — that is, to the focal distance. If this is not done, the impression is untrue. We have lenses with a focal distance of only four inches, and even less ; and at such a distance it is impossible to see a drawing with the unaided eye. To do this it must be held at the distance of at least eight inches, and that is the reason why photography in such Fig. 87. the picture is