Rationalizations

The Valley of Prisms got its name because the atmosphere had variable optical density such that light was bent and scattered while traveling through it.

Anybody who relied on vision alone would be doomed; unable to navigate properly and prone to suffering psychological damage and madness over time. Meanwhile, the lifeforms of the Valley of Prisms had evolved different senses, and their vision had dimmed. 

It spoke volumes that, despite this, the Divine Doctor had somehow been the one to drive the catoblepas insane instead!

The prism effect of the valley was known to come from its esoteric vegetation interacting with the atmosphere in such a way that the gaseous exchanges caused the optical density of the atmosphere to be entirely non-uniform.