Missing controller

You are seeing this error because controller ExhibitionController could not be found.

Notice: this error is being rendered by the app/views/errors/missing_controller.thtml view file, a user-customizable error page for handling invalid controller dispatches.

Fatal: Unable to load controller ExhibitionController

Fatal: Create Class:

<?php
class ExhibitionController extends AppController
{
    var $name = 'Exhibition';
}
?>

in file : app/controllers/exhibition_controller.php