Jesus made an apparently incongruous claim in the narrative for today. He referred to light as dark. I think the easiest way to comprehend His remarks is to contrast two different types of light.
In Matthew 6, Jesus warned those He was speaking to against pursuing wealth and urged them to store up their treasures in heaven. Then He used the metaphor of "the lamp of the body," the eye, to explain His point. If we keep our attention on spiritual matters, we will be radiant. However, if we pursue material wealth, we will fill with a dim light that He compared to tremendous darkness.