The final words of Inferno, describing Dante and Virgil's ascent out of the underworld, are as follows:
To get back up to the shining world from there
My guide and I went into that hidden tunnel;
[. . .]
Where we came forth, and once more saw the stars.
In these lines, the motif of light ("the shining world . . . stars") represents how Dante and Virgil, looking up to the heavens, can now see with the optimism of possible salvation as opposed to the pessimism of sin and despair that they became accustomed to in the underworld. Fundamentally, these lines illustrate how one's perception of the light is informed by one's perception of the dark.
From "The Second Coming," the lines quoted below are important:
When a vast image out of Spiritus Mundi
Troubles my sight: somewhere in sands of the desert
A shape with lion body and the head of a man,
A gaze blank and pitiless as the sun
By Spiritus Mundi Yeats means the collective soul or consciousness of the universe. He sees that this soul, or consciousness, is monstrous and "pitiless" (he wrote this poem in 1919, after World War I), and it is in seeing this that he realizes that all hope ("Surely the Second Coming is at hand") is lost. Thus, the creature which represents the soul of the world "troubles (his) sight" and changes how he sees the world and its future.