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"One evening coming in with a candle I was startled to hear him [Kurtz] say a little tremulously, 'I am lying here in the dark waiting for death.' The light was within a foot of his eyes. I forced myself to murmur, 'Oh, nonsense!' and stood over him as if transfixed." (3.41)
Kurtz’s madness is causing him to go blind; he cannot see the sunlight. He is completely immersed in the darkness and evil of his soul.
"He cried in a whisper at some image, at some vision – he cried out twice, a cry that was no more than a breath:
'The horror! The horror!'" (3.42-43)
Kurtz is having death visions and is raving. It is all fantastically and madly horrible.