Yelsgib:

Before Keats found out about what rainbows "really are" he experienced wonder while looking at them. After, he didn't.

What else is the man supposed to do? He's got to try to investigate his experience, right? Where did he go wrong?

I think the point Eliezer is trying to make is: The man should have investigated the rainbow scientifically and then feel wonder when he understood the physics behind it.