SENSE OF VISION. 891 



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 a single 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 Prof. 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 successive impressions, passing rapidly before the eye, give 

 rise by their combination to the idea that the object is itself moving through 

 these positions. 1 It is interesting to remark, moreover, that when the eye has 

 been for some time contemplating an object in motion, and is then directed 

 towards stationary objects, these appear for a short time to have a like movement. 

 Any railroad traveller may try this simple experiment, by first looking at the 

 hedges, &c., which he is rapidly passing, and then at some part of the interior 

 of the carriage itself, especially one which presents a series of parallel lines. 

 But when the impression of movement has been of longer duration, its effects 

 upon the retina are less transient ; thus a person who has been for some time 

 on board ship, sees the floors, walls, and ceilings of his apartment on shore 

 in a state of continual up-and-down motion, even for some days after he has 

 landed. 



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

 sion of some particular kind, it seems less susceptible of feebler impressions 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 colors in 

 succession these being expressions of the states through which the retina 

 passes, in its transition to the natural condition. If the eye has received a 

 strong impression from a colored object, the spectrum exhibits the complement- 

 ary color ; a thus, if the eye be fixed for any length of time upon a bright red 



1 A very beautiful " philosophical toy" was shown to the Author some years since, by its 

 inventor, Mr. Roberts, the celebrated machinist of Manchester ; consisting in an apparatus 

 by which it was made possible to read words printed on a card, although the card itself 

 was made to revolve on its axis even 30,000 times in a minute. The principle of its con- 

 struction was simply this that the eye caught a succession of glimpses of the card, 

 through a narrow slit before which a disk with a single corresponding perforation was 

 made to revolve; the rate of movement of this disk being so adjusted to that of the card, 

 that, whenever the eye caught sight of the latter, it was momentarily in the same position, 

 so that, by the succession of transient impressions thus made upon the retina, the words 

 printed on the card could be distinctly read. 



2 By the "complementary" color is meant that which would be required to make white 



