In Sonnet XVIII by William Shakespeare, the use of "eye of heaven" is a figure of speech known as metonymy, the substitution of something closely related for the thing actually meant. With the sun being this "eye of heaven," a greater meaning is then attached since the eye is the agent of perception and the indicator of character.
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd;
And every fair from fair sometime declines,
By chance, or nature's changing course, untrimm'd;
But thy eternal summer shall not fade,
The strong eye of the sun dims the gold complexion; that is, it hides the beauty and deprives the loved one of her fairness. But, the poet intends to give his love "eternal lines" that will prevent this loss of beauty. Unlike nature, which is often too harsh or ephemeral, the poet gives "eye" to his verse by declaring his love's beauty as such that "shall not faide" because its character is preserved in the lines of the sonnet, and thus, the poet provides eternal life to his love's beauty:
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.