Throughout the novella, the theme of duality is explored through the motif of light and darkness, most meaningfully through the symbol of the moon. In the account of Carew’s murder, the moon plays an important role:
Although a fog rolled over the city in the small hours, the early part of the night was cloudless, and the lane, which the maid’s window overlooked, was brilliantly lit by the full moon.
The moon overlooks the entire scene, as it does on the night that Hyde commits suicide. These are two of the most revelatory points of the narrative. Carew’s murder is the peak of Hyde’s criminal behavior, which ultimately results in his downfall. The night of Hyde’s suicide is when Jekyll’s relationship to Hyde is finally revealed.
Notice the fog that appears in this paragraph (“a fog rolled over the city in the small hours”). Everywhere obscuring fog or mist appears in the text, the moon appears as well. The accompaniment of one by the other echoes the ideas of good and evil in the book, effectively underscoring the idea that these things coexist side by side in most people.
But beyond this, the symbol of the moon actually echoes the duality present in Jekyll himself. The moon naturally is half-obscured to the observer’s eye; Jekyll obscures the ugliest parts of his character. The moon is unknowable, beyond the physical reach of scientists in this period. Likewise, parts of even seemingly good people, like Jekyll, are beyond the influence of reason and science.
