Tutorial Optical illusion with Flash (Actionscript 2)

// Creates an empty movie clip "mc" at depth 1
this.createEmptyMovieClip("mc",1);

// Center the movie clip on the stage
mc._x=mc._y=300;

// Set the line style : 5 pixels / black / alpha 100%
mc.lineStyle(5, 0x000000, 100);

// set the fill style : black
mc.beginFill(0x000000);

//draw the first twirl
for (i=0;i<=Math.PI*30;i+=Math.PI/30) mc.lineTo(Math.sin(i)*(i)*i/10, Math.cos(i)*(i)*i/10)

//draw the second twirl
for (i=Math.PI*30;i>0;i-=Math.PI/30) mc.lineTo(Math.sin(i)*(i)*i/9, Math.cos(i)*(i)*i/9)

//stop the filling
mc.endFill();

//rotate the movieclip
mc.onEnterFrame = function() {this._rotation+=20};