I just did some searching and found that 18A is the Wratten number of the transparent UV filter. For some reason I though it was 1A. So, there you have it.
BTW, interesting about the cornea filtering out UV that would otherwise be detectable to the retina. What color does the brain perceive UV as, did they mention? I’m guessing a deep violet.
Here’s some fun info
And the book is Kodak publication B-3, KODAK PHOTOGRAPHIC FILTERS HANDBOOK. Available here.
Figures UV would have a wratten #. I’ve never seen it printed on a threaded filter, tho. Most of the other colours are avaible in threaded glass filters from almost anyone in addition to the Kodak gels. The same #s are used for astronomy as for photography.
Sorry, Padeye. I’m not ignoring you, just missed your post in that last flurry of activity and board wonkiness.
Your point is well taken, thanks, but I think you know what I meant. 
Also, looks like I won’t need to post a comprehensive lsit, as everyone seems to be finding one on their own.
Board traffic must be high right now, so I’m just going to go over here for a little while…
[leaves to actually outside for a while]
Well, you and Eats_Crayons know what you’re talking about whereas I…
Well, I started this thread didn’t I.
Thanks everyone for all the great info.