Visual imagery helps establish the appearance of each character. In Volume 1, Chapter 1, Austen provides a detailed description of young Catherine:
She had a thin awkward figure, a sallow skin without colour, dark lank hair, and strong features;—so much for her person;—and not less unpropitious for heroism seemed her mind.
Details about Catherine's size, figure, and complexion reveal that she lacks the characteristics of a traditional heroine. She is not pretty, strong, or remarkable in any way. Em dashes interrupt the physical description and punctuate the facts about her appearance with the narrator's opinion.
In the same chapter, the narrator describes her five years later, at age 15:
At fifteen, appearances were mending; she began to curl her hair and long for balls; her complexion improved, her features were softened by plumpness and colour, her eyes gained more animation, and her figure more consequence. Her love of dirt gave way to an inclination for finery; and she grew clean as she grew smart; she had now the pleasure of sometimes hearing her father and mother remark on her personal improvement.
Here, Catherine's appearance begins to improve, and she becomes "clean" and "smart." These parallel instances of visual imagery give the reader a sense of Catherine's physical evolution. However, she remains somewhat average-looking, and her appearance never quite matches what one might expect from a heroine.
Austen uses visual imagery to give the reader a sense of the grandeur of Northanger Abbey. In Volume 2, Chapter 7, General Tilney describes his estate:
The house stands among fine meadows facing the south-east, with an excellent kitchen-garden in the same aspect; the walls surrounding which I built and stocked myself about ten years ago, for the benefit of my son. It is a family living, Miss Morland; and the property in the place being chiefly my own, you may believe I take care that it shall not be a bad one.
While the General does not use terribly descriptive adjectives (see "good" and "excellent"), he does list the relative locations of many rooms and features. The narrator provides an even more detailed description in the same chapter:
The number of acres contained in this garden was such as Catherine could not listen to without dismay, being more than double the extent of all Mr. Allen’s, as well as her father’s, including church-yard and orchard. The walls seemed countless in number, endless in length; a village of hot-houses seemed to arise among them [...].
Visual imagery, combined with a bit of hyperbole ("countless", "endless") reinforces the General's description. Catherine tends to overreact to new experiences, but she is rightly awed by the abbey's grandeur in comparison to anything she has seen before.
