Sequential fading prototype?

1

Okay, I have a few boxes, I want them to start out with an alpha value of 0, then after the first box alpha value reaches 75, I want the second one to start fading in, this is what I have so far, but they all fade in at the same time, can anyone point me in the right direction with this?


MovieClip.prototype.fadeIn = function() {
	if (this["b"+(i-1)]._alpha>=75) {
		this._alpha += 15;
	} else {
		this._alpha = 0;
	}
};
for (i=1; i<5; i++) {
	var box = this["b"+i];
	box._alpha = 0;
	/*b1.onEnterFrame = function() {
		if (this._alpha<=100) {
			this._alpha += 15;
		} else {
			delete this.onEnterFrame;
		}
	};
	*/
	box.onEnterFrame = function() {
		this.fadeIn();
	};
}