Are you familiar with the story of "N-Rays" and their discoverer, René Blondlot? (Skepdic (http://skepdic.com/blondlot.html)) I read about him in Paul Collins' book Banvard's Folly, an excellent book about 13 people who were hailed as geniuses for a time, but have since disappeared from history. Blondlot discovered and named N-Rays, which are light emanations from most things, but which can only be seen with almost-but-not-quite-direct viewing.

Blondlot's story is apparently most commonly described as a lesson in experimental design, because the N-Ray phenomenon fell apart in the face of a basic double-blind (well, let's say one-and-a-half, since Blondlot's assistant witnessed some manipulation of the apparatus) experiment, even though it was carried out without severe formality.

But I bring this up because Collins' book suggests that N-rays are an artifact of the cone/rod arrangement on the typical human retina, a compelling idea that I haven't seen on the web.