Possible Clues:

• We're not able to see or find about how rare this is
• This ray enables one to see in the dark
• Ray, perhaps, uncovering rare find
• Find around what is unusual at the end of the rainbow
• It's rare to be in the find but never in the spectrum
• Find out about unusual forms sensitive to some radiation
• Form of radiation
• Part of the invisible spectrum
• Kind of rays that enable one to see in the dark
• That's what you'tt find where the rainbow ends