If you stand at the bottom of a well and look up at the entrance is it possible to see stars in daylight?
The scattering of light in all directions by molecules in the Earth's atmosphere means that regardless of objects blocking direct light from the sun ( for example the walls of a well or the sides of a cardboard tube held up to the eye) a sufficient number of photons will enter the eye to 1) constrict the pupil, reducing total light entry, 2) cause rod photoreceptors in the retina to become "light-adapted" and thus less sensitive to low levels of light, and 3)provide a bright background and thus less contrast for cells in the retina and visual cortex that determine the edges of objects based upon differences in brightness. Due to these effects, the light from most stars remains indiscernible despite restricting the viewing angle.
However, the light from planets and even a few stars is bright enough to occasionally be detected during the day. So the answer to your question is yes, but realize that this is not some special property of wells, but just something which is occasionally possible anytime you limit the amount of light obliquely reaching the retina. Note that this detection of faint light sources is most difficult when light hits the fovea, which occurs when you look directly at something. Thus attempts to view faint light sources often results in a frustrating sense of catching a glimpse of the source in the peripheral vision, only to have it "disappear" when you try to look right at it.
Rob Landolfi, Science Teacher, Washington, DC
'We are all in the gutter, but some of us are looking at the stars.'