Has your dog been squinting more than usual? Is at least one of his eyes red? And is there some mucous discharge from one or both eyes - not the grey, jelly-like ooze that you can easily wipe away but something thicker and perhaps more yellowish as well as harder to remove? If so, get your dog to the veterinarian. Theres a reasonable chance he is not producing enough tears and has a condition called dry eye, known in medical circles as kerato-conjunctivitis sicca. Its a fairly common illness among dogs that can lead to impaired vision if not diagnosed and treated.