The poem plays a clever trick with its title. A spirit level is the builder's tool with a little bubble floating in liquid, and the "two vacant globes preserved in alcohol" are, at first, oddly like eyes in a jar. The speaker marvels that something so small could hold a construction worker's full attention, the same "devotional gaze" usually saved for breasts or buttocks that seem to defy gravity.
The tone is dry and teasing. The speaker pokes fun at the worker's habits, noting that he doesn't always look at the eyes of the people he ogles, and the "cool disregard" in those eyes becomes a kind of judgment. The final image turns the joke around: the worker squints and whistles at anything that "fails to fall into line," which fits both a crooked wall and a person he's sizing up. The mood is wry and observant, with a slightly uneasy edge, since the poem quietly links checking a surface with checking a body.