Symbolism, Imagery, Allegory
Chiaroscuro, anyone?
There's a lot of imagery throughout The Hours that combines light, shadow, and darkness in striking ways. Sometimes, those combinations help to characterize physical spaces, as in these examples:
Night still resides here. Hogarth House is always nocturnal, even with its chaos of papers and books, its bright hassocks and Persian rugs. It is not dark in itself but it seems to be illuminated against darkness, even as the wan, early sun shines between the curtains and cars and carriages rumble by on Paradise Road. (2.6)
His apartment is, as always, dim and close, overheated, full of the sage and juniper incense Richard burns to cover the smells of illness. It is unutterably cluttered, inhabited here and there by a wan circle of pulverized non-dark emanating from the brown-shaded lamps in which Richard will tolerate no bulb more powerful than fifteen watts. The apartment has, more than anything, an underwater aspect. (4.25)
It's no coincidence that these physical spaces are, respectively, the places where Virginia Woolf and Richard Brown reside. Virginia and Richard are both dealing with some serious inner demons, so it's appropriate, really, for to live in places that are shadowy and sometimes even a little bit scary.
Cunningham uses similar kinds of light, shadow, and darkness imagery to describe the illnesses that these characters live with (or have lived with) daily. Take a look:
Everything glows and pulses. Everything is infected with brightness, throbbing with it, and she prays for dark the way a wanderer lost in the desert prays for water. The world is every bit as barren of darkness as a desert is of water. There is no dark in the shuttered room, no dark behind her eyelids. […] This state makes her hellishly miserable; in this state she is capable of shrieking at Leonard or anyone else who comes near (fizzling, like devils, with light) […]. (5.4)
"Are they here today?" Clarissa asks.
"No," Richard answers, with the reluctant candor of a child. "They're gone now. They're very beautiful and quite terrible."
"Yes," she says. "I know."
"I think of them as coalescences of black fire. I mean they're dark and bright at the same time. There was one that looked a bit like a black, electrified jellyfish." (4.48-51)
So: What does all of this mean?
At heart, The Hours is a novel that explores the beautiful and terrible tensions between life and death. Light, shadow, and darkness imagery are pretty perfect symbols for representing those tensions visually; literally thousands of years' worth of literary and cultural tradition has trained us to associate darkness with death and light with life.
So, basically, by mixing both darkness and light together in captivating ways, Michael Cunningham draws attention to the ways in which life and death comingle in the minds of some of the novel's characters.