SENSE OF VISION. 253 



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so contrived, that its distance from the eye cannot be estimated in the ordinary 

 manner; the objects it represents are invested by the mind with their real 

 sizes and respective distances, as if their real image was formed upon the 

 retina.* 



343. 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 con- 

 vey. 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 any thing 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 pre- 

 cise conclusion on the subject, from the want of sufficient data, it is indubita- 

 ble that a large part of the knowledge of the external world, which he derives 

 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 sensations 

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



344. 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 only be 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 ; 

 otherwise a confused picture is produced. Thus in a little toy, called the 

 Thaumatrope, 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 in 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, how- 

 ever, 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 pur- 

 pose 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, pass- 

 ing rapidly before the eye, give rise by their combinations to the idea that 

 the object is itself moving through these positions. Similar illusions may be 

 produced in regard to colour. 



345. When the Retina has been exposed for some time to a strong impres- 



* This delusion has been extremely complete, in some of those who have seen the 

 panoramic view of London in the Coliseum. A lively and interesting account of it is 

 given in the Journal of the Parsee Shipbuilders, who recently visited England. 

 22 



