420 ON SENSATION, AND THE ORGANS OF THE SENSES. 



550. From all these considerations, we are led to perceive the truth of the 

 quaint observation made by Dr. Brown, that "vision is, in fact, the art of 

 seeing things which are invisible ;" that is, of acquiring information, by means 

 of the eye, which is neither contained in the sensations of sight themselves, 

 nor logically deducible from the intimations which those sensations really 

 convey. We cannot too constantly bear in mind, in treating of this subject, 

 that we do not take cognizance by our optic nerves, as we do by the nerves 

 of touch, of material bodies themselves, but of the pictures or images formed 

 by those objects; and whatever be the notions suggested by the picture, that 

 can never be transformed into anything else. These notions appear to be, in 

 the lower Animals, entirely of an intuitional or instinctive character; in Man 

 they are so in a much less degree ; and although it is impossible to come to a 

 precise conclusion on the subject, from the want of sufficient data, it is indu- 

 bitable that a large part of the knowledge of the external world, which he de- 

 rives in the adult condition from the use of his eyes alone, is really dependent 

 upon the early education of his perceptive powers, in which process, the sen- 

 sations conveyed by different organs are brought to bear upon one another. 



551. The persistence, during a certain interval, of impressions made upon 

 the retina, gives rise to a number of curious visual phenomena. The pro- 

 longation of the impression will be governed, in part, by its previous duration. 

 Thus, when we rapidly move an ignited point through a circle, the impression 

 itself is momentary, and remains but for a short time; whilst, if we have been 

 for some time looking at a window, and then close our eyes, the impression of 

 the dark bars traversing the illuminated space is preserved for several seconds. 

 Such phenomena can here be only briefly adverted to. One of these is the 

 combination, into one image, of two or more objects presented to the eye in 

 successive movements ; but these must be of a kind which can be united, other- 

 wise a confused picture is produced. Thus in a little toy, called the Thauma- 

 trope, which was introduced some years ago, the two objects were painted on 

 the opposite sides of a card, a bird, for instance, on one, and a cage on the 

 other; and, when the card was made (by twisting a pair of strings) to revolve 

 about one of its diameters, in such a manner as to be alternately presenting 

 the two sides to the eye at minute intervals, the two pictures were blended, 

 the bird being seen in the cage. A far more curious illusion, however, was 

 that first brought into notice by Mr. Faraday ; who showed that, if two toothed 

 wheels, placed one behind the other, be made to revolve with equal velocity, 

 a stationary spectrum will be seen; whilst if one be made to revolve more 

 rapidly than the other, or the number of teeth be different, the spectrum also 

 will revolve. The same takes place when a single wheel is made to revolve 

 before a mirror ; the wheel and its image answering the purpose of the two 

 wheels in the former case. On this principle, a number of very ingenious 

 toys have been constructed; in some of these, the same figure or object is 

 seen in a variety of positions; and the impressions of these, passing rapidly 

 before the eye, give rise by their combination to the idea, that the object is 

 itself moving through these positions. Similar illusions may be produced in 

 regard to colour. 



552. When the Retina has been exposed for some time to a strong im- 

 pression of some particular kind, it seems less susceptible of feebler impres- 

 sions of the same kind. Thus, if we look at any brightly luminous object, 

 and then turn our eyes on a sheet of white paper, we shall perceive a dark 

 spot upon it; the portion of the retina, which had been affected by the bright 

 image, not being able to receive an impression from the fainter rays reflected 

 by the paper. The dark spectrum does not at once disappear, but assumes 

 different colours in succession, these being expressions of the states through 

 which the retina passes, in its transition to the natural condition. If the eye 



