How does Fitzgerald differentiate between image and substance?
Consider the first encounter Nick has with Jordan Baker, when he visits at Tom and Daisy Buchanan's home. Fitzgerald's initial description of Jordan and Daisy is filled with impressions and images.
an enormous couch on which two young women were buoyed up as though upon an anchored balloon....their dresses were rippling and fluttering as if they had just been blown back in...
It takes several more pages before Fitzgerald provides any concrete description of Jordan Baker's actual physical appearance. He does explain that she moves like the athlete she is and that she seemed vaguely familiar to Nick even before he confirmed why he recognized her face, having seen it in photographs of sporting events.
Throughout The Great Gatsby, Fitzgerald uses passages of impressionistic description to set mood and paint pictures of the emotions and feelings being experienced by the characters as well as specifically describing the substance and reality of people, places and events.