Stupid Typescript slide show question

class Greeter {
    element: HTMLElement;
    span: HTMLElement;
    timerToken: number;
    constructor(element: HTMLElement) {
        this.element = element;
        this.element.innerHTML += "The time is: ";
        this.span = document.createElement('span');
        this.element.appendChild(this.span);
        this.span.innerText = new Date().toUTCString();
    }
    start() {
        this.timerToken = setInterval(() => this.span.innerHTML = new Date().toUTCString(), 500);
    }
    stop() {
        clearTimeout(this.timerToken);
    }
}
window.onload = () => {
    var el = document.getElementById('content');
    var greeter = new Greeter(el);
    greeter.start();
    var e2 = document.getElementById('img1');
    var ssimg1 = new slideshow(e2);
    ssimg1.img1_start();
};
class slideshow {
    _index: number;
    _img1_element: HTMLElement
    _img1_count: number;
    _asfiles: Array<string>;
    _tmrtkn: number;
    constructor( img1_element: HTMLElement) {
        this._asfiles = ['src="img1/IMGP0675.jpg" width="1024" height="684"',
            'src="img1/IMGP1094.jpg" width = "1024" height ="684"'];
        this._img1_element = img1_element;
        this._img1_count = 0;
        this.img1_write();
    }
    img1_write() {
        this._img1_count = this._img1_count + 1;
        this._index = this._img1_count % this._asfiles.length
        this._img1_element.innerHTML = "<img alt=xmess " + this._asfiles[this._index] + "\>";
               // document.
    }
    img1_start() { this._tmrtkn = setInterval(this.img1_write(), 3000);}
    img1_stop() { clearTimeout(this._tmrtkn);}
};