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620 producing a distortion similar in kind, but far greater in degree, than the barrel-shaped distortion of the image made by an ordinary single lens when used with the stop in front. Whichever way we regard the case, it is simply impossible that a plane surface, whether of a picture to be copied, or of a building to be photographed, can be truly reproduced by an image received on a surface which is not also plane* and parallel. As nothing, perhaps, in the whole theory of vision is more certain than the fact that we are not conscious of the whole image on the retina as such, seeing, as we always do, the upright object—not the inverted image—the lines of a building as rectilineal, and not as curved, like the image formed on the retina : that fact scarcely needs the proof afforded, really, though not formally, a mathematical demonstration ; that our visual perception, though neces sarily based on sensation, does not necessarily coincide with it. As we shall endeavour to show, the visual image owes its form—perhaps even more than its structure—rather to the mental processes of a combining memory, and an intellectual interpretation of the signs presented to us through sensa tion, than immediately to the sensation itself. We have seen that the image of a plane surface will always be correctly represented as to form when depicted on another plane surface parallel to it; and that vertical lines are then parallel in the photograph as in the building itself. Painters also represent them as parallel in their pictures, and architects draw them so in their plans. I may too, I think, safely appeal to our common experience, that we always see them so in natural vision, and expect so to see them in any pictorial representation. But how does it come about that we do so see them, such vision not agreeing with the apparent angles subtended, nor with the image on the retina 'I The answer may properly be, because it does correspond very precisely with the true form of the object, and is a striking example of the fact that it is true form, rather than apparent form, which habitually dominates our visual perceptions. Though, however, we thus speak of the form that actually exists— the tangible form—as true form, it is right we should bear in mind that neither visual nor photographic pers pective are properly matters of truth or of falsehood. The actual (or tangible) form of an object is one true fact; the (so-called) apparent form, corresponding to the angles subtended, and necessarily varying with varying distance and position, is another true fact. The two are quite incom patible in any single physical representation; but in the mental image which is the true visual perception, their differ ences tend to melt into one another, according to the vary ing strength of dominant preconceptions ; and as the eye “ wanders ” over the whole, running along vertical lines, and recognising their true verticality; along horizontal lines, and recognising them asactually horizontal; and recognising also the actual parallelism of successive pairs of lines, our mental image of the whole scene is gradually built up. It is thus that a compromise is arrived at between our judgments of apparent magnitude and our estimates of real magnitude, as well as of our perceptions of apparent form, and our knowledge of true form. It is thus also that we appre hend distance, inferring it from the various signs presented to us in sensation, and uniting with it such conception of magnitude in distant objects as invests our perception of them with a dignity which would not otherwise be asso- * We of course leave out of consideration the unpractical case of a distorted image received on a distorting surface so contrived, as by opposing action, to correct one another, That would be only a toy. ciated with the small angle under which they are seen. And it is thus—our gaze dwelling and lingering on the most interesting objects, while passing more rapidly over others —that the whole scene gathers order and purpose. Selec tion and emphasis arise in the mind, and we gradually learn to make pictures for ourselves. Finally, it is thus that a just sense of perspective is, to a perceiving mind, the latest resultant of its intellectual and emotional forces, modifying, as they interpret, the visual sensations. The mental process above-described may seem long and complex, quite absurdly long for an “ impression ” ; but “thought is quick.” This is not all, however. Our practical results are probably, in general, far less those of direct present inferences than of an association of ideas resulting from previous inferences, and thus the general result may be hastened and simplified. These mental processes, too, being rarely accompanied by any conscious ness, we—thinking of the objects we are looking at, and not at all of the process by which we recognise them—are not apt to realise that process. Still, the common forms of speech, “Cast your eye over it,” “Run your eye along it,” do seem an evidence of some general practical recog nition of the means by which we habitually judge of form and magnitude. Perhaps I ought here to remark that it is necessary, in dealing with this subject, to distinguish more carefully between the strict meanings of the words “ sensation” and “ perception ” than is at all necessary in their common use. “Sense-perception” is a not-unfrequent phrase, and the similar one “ sensuous impression,” may serve very well to express the effect of a hasty glance, in which feeling is-supposed to predominate over the results of an intellec tual process, though whether feeling or emotion be not itself a product rather of the mind than of the body— rather of perception than of mere sensation—may well be doubted.* Perhaps, like a chemical combination, it should rather be looked on as a new product. I am not clear, however, as to what is the idea generally associated with the word “ impression.” Of course an impression on the eye really means an impression on the brain, which is to us a mental impression. Professor Max Muller remarks that M. Flourens proved by experiments that “if the roots of the optic nerve are removed, the retina in the eye of a bird ceases to be excitable, the iris is no longer movable ; the animal is blind, because it has lost the organ of sensation. If, on the contrary, the cerebral lobes are removed, the eye remains pure and sound, the retina excitable, the iris movable. The eye is preserved, yet the animal cannot see, because it has lost the organs of per ception. t I see that I have left myself no space for the wide difference between the relations of visual and photographic perspective when taking a straight and symmetrical view of a building, from those which exist when, on the other hand, we direct our gaze and our camera obliquely— especially if from so near a point of view as to necessitate the use of a wide-angle lens—and must postpone that to another opportunity, if space can be found for me. I may say, however, that the same sense of real form which corrects the distortion shown necessarily to characterise crude sensation in the straight and symmetrical view—could * Hartmann explains feeling as resulting from affections of the will in combination with conscious and unconscious ideas.— Ueberweg. Hist. Phil., vol. ii., p. 336. + Science of Language t vol. i., page 389.