The dominant sense in Hoffmann's “The Sandman” is sight. One of the main characters in the story, Coppola, sells spectacles; Nathanael apprehends the world primarily through his eyes; and Olimpia's status as a puppet is revealed when Coppola removes her eyes.
Then of course, we have the eponymous Sandman, a mythical creature who creeps into children's bedrooms at night and throws sand in their eyes if they don't go to sleep.
At almost every stage of the story, there is a notable gap between what the eye can see and what's really going on. For example, Nathanael thinks he can see his father turning ugly in the presence of Coppelius. But it's by no means certain that this is the case at all. One gets the impression that Nathanael's boyish imagination is working overtime.
Later on, when Nathanael has grown into an adult, his senses seem to play tricks on him once more when he falls in love with what he believes to be a beautiful young woman called Olimpia. As it turns out, Olimpia isn't a woman at all but a large puppet. Nathanael only thought she was a beautiful woman because he was looking at her through a glass sold to him by Coppola, the man he thinks is really Coppelius.
Olimpia may have a fine pair of eyes, but they're not real, and Coppola doesn't hesitate to pull them out and leave them behind when he steals the puppet from Spalanzani, Olimpia's creator. The eyes contributed in no small part to the illusion that Olimpia was a real woman and which fooled Nathanael completely at first.
If there is one message to be derived from all of this, is that's our senses can often play tricks on us, can distort reality, and make us believe all kinds of strange things.