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Miscellaneous Intelligence. 133 



If the blue spot is now examined in the same manner, it will pre- 

 sent an appearance somewhat different, the borders that form on its 

 edge being of a lighter color than the rest of the spot, and having ihe 

 effects of lights instead of shadows, as in the former instance. 



These changes of tint are so momentary, that they require very close 

 attention in order to be seen at all ; and the best way of discovering 

 them is to hold the pattern at some distance from the lamp or candle, 

 moving it at first rather slowly. 1 have described these appearances 

 thus minutely because they serve, according to my observation, to pro- 

 duce the illusion in question : in them lies the whole mystery. 



I conceive that the light and dark borders in the two figures have the 

 same effect on the mind of the observer as the ordinary light and shadow 

 seen on any prominent object that is illuminated from one side ; and if 

 such an object could be presented to our view with its light and shade 

 shifting rapidly from one side of it to the other, though without its being 

 moved, I believe we should have an impression that the object really 

 was moving, because we know from experience that such an effect is 

 °nly to be produced by shifting the position of the candle, or of the 

 object illuminated by it.* 



And so it is with the worsted patterns. When we see these spots 

 with their light and dark borders shifting from side to side, we are led 

 to infer that, as the lamp or candle is stationary, the spots themselves 

 actually shift their position, and move faster than the ground of the 

 Pattern. Such at least is the illusion ; and in some instances the spots 

 will even appear like protuberant bodies rolling from side to side. 

 b In order to prove that this change of tint at the edges of the figures 

 is an efficient cause of the illusion, I prepared a small instrument on 

 this principle. A blue and red spot were affixed to a white card in 

 such a manner that movable shadows might be introduced behind 

 them, which were made to slide from side to side in imitation of the 

 borders of the worsted spots, and to persons not acquainted with the 

 nature of the contrivance the effect was the same, the colored spots ap- 

 peared to move when their shadows only were being moved. 



On the other hand, when the formation of the light and dark bor- 

 ders is prevented, as may be done by inserting a row of white stitches 

 round each of the spots"so as to separate the two colors, then the illu- 

 sion ceases — the figures are no longer seen to move. The reason of 

 this is obvious, that the presence of the white stitches is an obstacle to 

 the blending of the colors on which the light and dark borders depend 

 for their production. The difference of tint in the two cases may re- 

 hire some further explanation, — why the red spot has a dark border, 

 an d the blue a light one. The apparent blending of colors in a state 

 of rapid motion is generally accounted for by the physiological fact, 

 that the eye retains impressions for an appreciable time after the ob- 



* I have often convinced myself of the truth of this hy li Mgk experiment. 

 Some prominent body, as a white ball or cylinder, IS tuspem ed before an observer 

 seated with bis back to a table, on which two lighted candles are placed at some 

 Stance apart By covering up each candle alternately, the light and shadow on 

 the object may be reversed ; and if this is done rapidly, it will give it the appear- 



ance of moving from side to side. 



