It faced – or seemed to face – the whole external world for an instant, and then concentrated on you with an irresistible prejudice in your favor.
Most of this paragraph is taken with a description of Gatsby’s smile. That smile is a wonderful piece of work, and Gatsby must have truly worked at it – you can picture him spending hours in front of the mirror perfecting the radiant character of that smile. He started with natural material, the handsome face, a wide mouth around even white teeth. His ambition to become a man of the world required an openness to experience beyond his small beginnings, and that open heart can fill a smile with good will. He continually nurtured his ability to appreciate and reflect joy, if not always to generate it. All of that could be genuine.
What could not be genuine, what does require that practice in the mirror, is the way that smile turns to just you, no longer the natural sun hanging in the sky buy a spotlight made and tailored just for you. You have to remember, that’s how he smiles to everyone, it’s not just for you. There’s a particular technique in this, a conscious effort required to give you that irresistible prejudice. He has to lean into you a degree or two more, focus both of his eyes into your dominant eye, hold the gaze a fraction of a second longer than normal. That smile is a work of art, not a work of nature.