Some critics think that Milton's blindness gave him an uncanny ability to depict light, darkness, and shadow. This sonnet offers pretty strong evidence for that claim. The central extended metaphor combines the concepts of light and money into symbolism as dense as seven-layer cake. Light and darkness interact in strange ways in Milton's poems. One of the most famous sections in Paradise Lost describes the interior of Hell as having:
No light; but rather darkness visible
Served only to discover sights of woe,
"Darkness visible." Wow. Milton's blindness proved to him that one can "see" even without light. We should also point out that the distinction between light and darkness is central to Christian theology.