int circleSize = 20;
int numberOfPositions = 100;
int []mousePosX = new int[numberOfPositions];
int []mousePosY = new int [numberOfPositions];
void setup()//
{
  size(842, 480);
}

void draw()//
{
  clear();
  background(0);
  addMousePosition();
  for (int i = 0; i < mousePosX.length; i++) {
    // alpha will define the transparancy (alpha).
    // The number of steps between 255 and 0 will be inline with number of elements in the array.
    float alpha = map(i, 0, mousePosX.length, 255, 0);
    fill(255, alpha);
    ellipse(mousePosX[i], mousePosY[i], circleSize, circleSize);
  }
}

void addMousePosition() {
  noStroke();
  fill(255);
  // Shift the array backwards, so the current mouse position can be added to
  // the array on index position 0
  for (int i = mousePosX.length-1; i > 0; i--) {
    mousePosX[i] = mousePosX[i-1];
    mousePosY[i] = mousePosY[i-1];
  }
  mousePosX[0] = mouseX;
  mousePosY[0] = mouseY;
}