Jail and containment
The primary aspect of imagery in the narrative is the depiction of containment and structure from the point of view of the poet trapped in a literal jail cell. The imagery inspires the intense emotional responses from Thoreau that makes him want to express himself, because he feels claustrophobic and contained. By choosing to deliberately subject himself to the jail sentence, he literally illustrates the power of the government, which helps his political activism.
Darkness and the sublime
The sublime is depicted in the murky, dark imagery that prevails. Much of the action happens at night, as the title implies, and much of the scenery is portrayed with darkness, and the texture lends a sublime feel to the book. This underscores the creepiness of transcendentalism, because the idea that there is meaning and intention lurking in the dark helps to stoke curiosity and imagination. The nighttime imagery also captures the troubling nature of Thoreau's situation.
Freedom and nature
The state of nature is an unignorable aspect of the text because Thoreau's obsession with nature is so well documented, as is evident also in his writings. But here, the use of natural imagery underscores the natural freedom of Thoreau. In spirit, he finds he is truly just an animal. The imagery provides a nice contrast to the containment and design of man made authority structures.
Dream imagery
A significant portion of the storyline takes place in Thoreau's dreams. Because he is physically trapped, his dreams begin to go in strange directions, problem solving perhaps, or perhaps just compensating for the monotony of life in jail. The dreams are described somewhat fantastically, but they pertain to real people, and they have grave, sometimes absurd imagery that typically points to the supernatural. The imagery helps the reader to understand Thoreau's day life within the context of his emotional and imaginative experiences.
