34-6 Mr. H. Taylor on the Apparent Motion of the Figures 



The usual patterns consist of blue and red figures on a 

 ground of the opposite colour several of them being arranged 

 together on the same piece ; but for the purpose of experiment, 

 it is better to have patterns of the simplest kind, and I shall 

 suppose them now to consist of single spots, of red on a blue 

 ground and blue on a red ground. 



If the pattern is moved up and down before the eye, and 

 attention is directed to the red spot, we may observe a change 

 of colour taking place at its edge, like a dark border or shadow 

 flitting from one side to the other. On close examination, it 

 will be found that as the pattern is moved upward, this dark 

 border appears only at the upper edge of the spot, and but 

 for an instant ; as the pattern descends, it is seen again at the 

 lower edge, vanishing as before. When the movement is 

 made from side to side, instead of up and down, a similar ap- 

 pearance may be seen on the right and left of the spot. 



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

 will present an appearance somewhat different, the borders 

 that form on its edge being of a lighter colour than the rest 

 of the spot, and having the effect 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. I 

 have described these appearances thus minutely because they 

 serve, according to my observation, to produce 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 ordi- 

 nary 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 only 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 



* I have often convinced myself of the truth of this by a simple experi- 

 ment. Some prominent body, as a white ball or cylinder, is suspended 

 before an observer seated with his back to a table, on which two lighted 

 candles are placed at some distance 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 appearance of moving from side to 

 side. 



