أَفَسِحْرٌ هَـٰذَآ أَمْ أَنتُمْ لَا تُبْصِرُونَ
Afasihrun haazaaaa am antum laa tubsiroon
Is this magic, then, or do you not see?
Sihr is the standing charge brought against revelation by those who reject it, at 34:43, 46:7 and 37:15, and the question here returns their own word to them with the fire in front of them. The demonstrative carries over from the line before, so what was named as the thing they denied becomes the subject of their accusation. Failure of sight is offered as the only alternative, and clear sight was part of what they had claimed.