Step 1Cook the bacon in a 4-quart pot over medium-high heat, stirring often, until the fat is rendered and the bacon is crisp, about 3 minutes. Discard all but 1 tablespoon of the fat.
Step 2Add the onion, celery, thyme, salt and pepper to taste. Cook until the onion is softened but not browned, stirring often, about 4 minutes. Stir in the flour. Cook 1 minute.
Step 3Slowly stir in the clam juice. Cook, stirring, until warm, about 2 minutes. Add the potatoes. Bring to a boil, then reduce the heat and simmer, covered, stirring occasionally, until the potatoes are tender, about 20 minutes.
Step 4Stir in the milk and cook until hot but not boiling. (This can be made a day ahead to this point and refrigerated. To serve, gently reheat the soup until hot. Add more milk if the soup is too thick.) Stir in the corn. Cook 1 minute. Stir in the scallops. Cook, stirring often, until the scallops are just opaque, about 2 minutes. Taste; adjust the seasoning. Serve hot, garnished with parsley or chives, if using.