Using a sharp knife, cut the corn kernels from the cobs. In a heavy casserole, cook the bacon over moderately low heat until slightly crisp, 3 to 4 minutes. Add the onion and celery and cook until softened, about 4 minutes. Add the potatoes, chicken stock, thyme and bay leaf. Season with salt and pepper and bring to a boil. Cover and simmer over low heat until the potatoes are just tender, about 10 minutes. Add the milk and simmer for 5 minutes. Add the corn and simmer until tender, 3 to 4 minutes. Remove the bay leaf and season with salt and pepper.