Again, even when there is no error of recognition, in the sense of confusing one object with other objects, there may be partial illusion. I have remarked that the process of recognizing an object commonly involves an overlooking of points of diversity in the object, or aspect of the object, now present. And sometimes this inattention to what is actually present includes an error as to the actual visual sensation of the moment. Thus, for example, when I look at a sheet of white paper in a feebly lit room, I seem to see its whiteness. If, however, I bring it near the window, and let the sun fall on a part of it, I at once recognize that what I have been seeing is not white, but a decided grey. Similarly, when I look at a brick viaduct a mile or two off, I appear to myself to recognize its redness. In fact, however, the impression of colour which I receive from the object is not that of brick-red at all, but a much less decided tint; which I may easily prove by bending my head downwards and letting the scene image itself on the retina in an unusual way, in which case the recognition of the object as a viaduct being less distinct, I am better able to attend to the exact shade of the colour. Nowhere is this inattention to the sensation of the moment exhibited in so striking a manner as in pictorial art. A picture of Meissonier may give the eye a representation of a scene in which the objects, as the human figures and horses, have a distinctness that belongs to near objects, but an apparent magnitude that belongs to distant objects. So again, it is found that the degree of luminosity or brightness of a pictorial representation differs in general enormously from that of the actual objects. Thus, according to the calculations of Helmholtz,[44] a picture representing a Bedouin's white raiment in blinding sunshine, will, when seen in a fairly lit gallery, have a degree of luminosity reaching only to about one-thirtieth of that of the actual object. On the other hand, a painting representing marble ruins illuminated by moonlight, will, under the same conditions of illumination, have a luminosity amounting to as much as from ten to twenty thousand times that of the object. Yet the spectator does not notice these stupendous discrepancies. The representation, in spite of its vast difference, at once carries the mind on to the actuality, and the spectator may even appear to himself, in moments of complete absorption, to be looking at the actual scene. The truly startling part of these illusions is, that the direct result of sensory stimulation appears to be actually displaced by a mental image. Thus, in the case of Meyer's experiment, of looking at the distant viaduct, and of recognizing an artistic representation, imagination seems in a measure to take the place of sensation, or to blind the mind to what is actually before it.