I've looked at the fly's origins and there as with so many flies, it seems shrouded in a bit of mystery. Some attribute it to George Griffith, one of the founding members of Trout Unlimited, others attribute it to an Andy Griffith - a sherif. Other sources credit a Walt Shaw with the invention, and George Griffith with its popularity.
What seems to evident is that it's an old pattern dating back at least to the 1930ies.
It's a midge imitation and thus should be tied small. I tie mine in sizes from 10 to 20, and they all catch well. The small ones definitely do a good job of imitating midges and the larger ones? Who know - terrestrials? Anything, really. It's one of those "get-out-of-jail-flies" that just sometimes work when nothing else does.
Check Thomas' book and video (link to iTunes) - it's an easy one to tie, and quite probably the only fly where I ever use those coveted grizzly dry fly hackles. I just don't use them, and I had three dry fly grade necks. Now two of them are dyed (dark olive and dark dun) making them muych more useful in imitations. There aren't too many insects out there with black and white legs.
I tie mine on a gold hook, but that's just for show - makes no difference and I like the hook - that's it.
I sometimes flush cut the underside of the hackle to allow the fly to sit more into the surface film. A very light application of gel is often a good idea, and sometimes, a sunk Griffith's Gnat fishes really well too.
If you haven't got any in the box - tie some. You can tie up a batch in no time and they catch fish.