Objects And Scope

In the method nextImage, I can't figure out how to access thumbs. It keeps coming back as undefined. (Using Firefox)

function runPortal(portal_number){
    // there are multiple runPortals on each webpage
    this.portal = document.getElementById('portal'+portal_number); // represents the div that holds the images
    this.thumbs = this.portal.getElementsByTagName('a').length; // represents all the images within the div that will be rotated
    this.length = this.thumbs.length; // that's how many images will be rotated
    // Hide everything
    for (var j=0;j<this.thumbs.length;j++){
        if (j==0) continue; // Don't hide the first one
        this.thumbs[j].childNodes[0].style.display = 'none'
    }
    this.nextImage = function (){
        // there are a fixed number of images to rotate. Start over
        if (this.i >= this.length){
            this.i = 0;
        }
        // One fades away, the next appears
        Effect.dglPuff(this.thumbs[this.last].childNodes[0], {duration:.6, from:.7});
        Effect.Appear(this.thumbs[this.i].childNodes[0]);
       
        // iterate to the next image for the next run
        this.last = this.i;
        this.i++;
    }
    // Set up the image rotator
    // here is where I started guessing
    // thumbs needs to belong to the object rotator, I guess.
   
    this.rotator = new PeriodicalExecuter(this.nextImage, 4); // This object runs the function every 4 seconds
    this.rotator.portal = document.getElementById('portal'+portal_number); // represents the div that holds the images
    this.rotator.thumbs = this.rotator.portal.getElementsByTagName('a'); // represents all the images within the div that will be rotated
    this.rotator.length=this.length;  // that's how many images will be rotated
    this.rotator.i=0; // the counter for what image we're one
    this.rotator.last=0; // the counter for the previous image
   
}