The last sentence the first paragraph (chapter five of Mary Shelley's Frankenstein) is as follows:
I saw the dull yellow eye of the creature open; it breathed hard, and a convulsive motion agitated its limbs.
This description of the creature's first breath, first movement, is meant to horrify the reader. Shelley, as denoted in the 1831 preface to the novel, stated that she desired to frighten her reader in the same way she was frightened when her nightmares created the "student of unhallowed arts" and his creation.
The sentence is filled with descriptive language: dull, yellow, hard, convulsion, agitated. Given that Shelley does not provide a complete description of the creature (limiting it to the height (around eight feet) and superficial elements of the yellow eyes, stretched skin, white eye sockets, and black lips), readers are left to create their own picture of the creature. Shelley's lack of a concrete description allows readers to create the most horrific image of the creature (limited only by their own imagination and creativity).
The atmosphere is, therefore, heightened given the limited nature of the description provided. Readers are already aware of Victor's fear.
It was already one in the morning; the rain pattered dismally against the panes, and my candle was nearly burnt out.
Therefore, not only is the scene set to be dark and dreary, the image of the yellow eye peering out from the shadowed room adds to the already frightening scene.