There is an excellent article in the July issue of Shooting Sports USA, about things that the eye sees while looking through an aperture sight/ peep sight. It is great stuff and explains why we sometimes see "floaters", "shrimp", and or cobwebs in the rear sight. It is a quick read and also explains why some rear aperatures work for some folks but not for everyone. If you get such a question on the line- you will know why and what they are seeing.
Thanks Spartan!
http://www.nxtbook.com/nxtbooks/nra/ssusa_201207/#/24 (http://www.nxtbook.com/nxtbooks/nra/ssusa_201207/#/24)
Thanks for the link to the article UL! It's a very interesting read.