This is a photo showing some cut cacti on the drying rack. Most have a 1/8" glowing ring right beneath the skin, which is the area we know has the highest concentrations of M. Some specimens don't glow at all, and some glow all the way to the core. If ones focus was to cultivate plants with the highest medicinal value, this method could be helpful in choosing which cacti to focus your attention.

Test your theory with extractions, maybe you can develop a reliable estimation method.

Is this with a standard "disco blacklight", or some other type of UV lamp?

It is not a guaranteed assumption that degree of fluorescence directly correlates with mescaline content. This is definitely something that should be followed up with quantifications. There are plenty of other fluorescent phytochemicals.

Interesting observation nonetheless!

It's a UV flashlight
https://www.amazon.com/F...Eliminator/dp/B01A5KLUG2

any compound with a conjugated pi-bond system will absorb even 365 nm UVA light (commercial blacklight blue). Mesc fits the bill, it has a benzene ring. Even limonene absorbs 365 nm light, and fluoresces slightly. Xylene fluoresces more strongly.

just keep in mind that even some structural proteins will fluoresce with 365 nm light, because they contain aromatic residues (tyrosine, phenylalanine and tryptophan)