用户看不到ModalViewController

_webView = [[UIWebView alloc] initWithFrame: CGRectMake(0, 0, 480, 320)];
[_authController addSubview:_webView];

[self presentModalViewController:_authController animated:YES];

NSLog([NSString stringWithFormat:@\"frame = %f, %f, %f, %f\", _webView.view.frame.origin.x, _webView.view.frame.origin.y, _webView.view.frame.size.width, _webView.view.frame.size.height]);

if (_webView)
{
    [self presentModalViewController:_webView animated:YES];
}

NSLog([NSString stringWithFormat:@\"frame = %f, %f, %f, %f\", _webView.view.frame.origin.x, _webView.view.frame.origin.y, _webView.view.frame.size.width, _webView.view.frame.size.height]);