The apostrophe is a literary device used in poetry and other literary works in which the speaker or character addresses a person, abstract idea, object or concept that is absent and in the case of ideas, these are intangible or nonhuman. This occurs in the case of Portia as she says "O constancy, be strong upon my side" because she is addressing an abstract idea which is "constancy" despite this is just an idea and cannot be present or tangible by definition. Thus, the form of figurative language in this example is the apostrophe.
