ACHILLES AND THE TORTOISE. 



Now let the electrician cause the three boards 

 A B C to turn over suddenly all together, so 

 as to show their red side. Similarly, let the 

 three next, D E F, be turned over immediately 

 afterwards, and in quick succession G H I. A 



FIG. 61. DISCONTINUITY OF MOTION. 



spectator on the mountain would now see that 

 a small red square has suddenly appeared near 

 one edge of the field ; or, being too far off to 

 see the exact shape of the square, he might 

 note simply that he saw a small red spot. 



Immediately after G H I have turned 

 their red sides up, let J K L do the same, and 

 simultaneously let A B C be turned over again 

 so as to show their green sides once more. 

 Then let M N O be turned to red and D E F 



279 



