Final answer:
The tone shift in Sonnet XIX occurs from a pessimistic octave to an optimistic sestet, with a volta marking the turn in thought. This structure is characteristic of sonnets, where the sestet often resolves or reflects on the octave's tension.
Step-by-step explanation:
The change in tone between the octave and the sestet of Sonnet XIX is best described as the octave being pessimistic, while the sestet is optimistic. This shift is commonly seen in sonnet structures, where the octave typically introduces a problem or sets a gloomy, serious, or pessimistic tone, which then sees a transition or volta. The sestet follows this turn and generally presents a resolution, reflection, or shift to a more positive or hopeful tone. The volta often occurs at the beginning of the sestet but can also appear earlier, signaling the change in direction of the sonnet's argument or theme.
As illustrated in Design by Robert Frost, a sonnet utilizing the Italian (or Petrarchan) form, there is an expected pattern. The octave presents a situation that is often dark or questioning, such as the white spider killing a white moth. The volta then introduces a shift, leading to the sestet, which explores the implications or answers the questions posed in the octave. In Frost's case, this turn doesn't lead to resolution but instead, further contemplation. Nevertheless, the change in tone anticipated at the volta is a defining characteristic of sonnet structure, whether the resolution is clear-cut optimism or remains contemplative.