static int circleSize = 30;
static int screenSize = 500;
static int maxFades = 20;
int[] miceX = new int[maxFades];
int[] miceY = new int[maxFades];
int intensity = 0;

void setup () 
{
  size(screenSize, screenSize);
  noStroke();
  smooth();
  fill(255);
}

void draw()
{
  background (10);
  /*This code grabs the new mouse position, and bumps the array 
  of old mouse positions "forward", starting from the back and
  pushing each value up one place in line.*/
  miceX[0] = mouseX;
  miceY[0] = mouseY;   
  for (int i = maxFades-1; i>=1; i--)
  {
    //print(miceX[i]+".");
    miceX[i] = miceX[i-1];
    //print(miceY[i]+",");
    miceY[i] = miceY[i-1];
  }
  
  println();
  //This sets the number of circles drawn based on the y position.  
  intensity = (miceY[0] / (screenSize/(maxFades-1)));
    //This is the brightness in each drawn circle, set by default to "full".
  int circleAlpha = 255;
  //This loop draws the circles, as many as are needed.  */
  for (int i=intensity; i>=0; i--)
  {
     //The greyscale will be exponentially darker in later circles.
     //It looks better that way.  
     circleAlpha = 255/(i+1);
     // println(circleAlpha);
     fill(circleAlpha);
     ellipse (miceX[i],miceY[i],circleSize,circleSize);
  }

}