At 02:16 PM 5/28/2011, Mark Iverson wrote:
It is widely known that the 'rod' cells in the retina, which are
responsible for seeing in low light
levels, are more concentrated just outside the center of focus, and
the 'cone' cells primarily
responsible for color vision, are more concentrated in the center of
the retina. Thus, one can
better see faint objects at night by looking slightly off center...
How many reading here have used a spinthariscope? In setting myself
up to do a little work, I bought some odds and ends, cheap stuff, and
one of the things I bought was a piece of CdS-coated plastic, which,
with an 8X hand microscope, which I have, makes a handy
spinthariscope, and I've spent some time at night looking at the
flashes of light from an Am-241 source ripped from the guts of an old
smoke detector.
I've spent a fair amount of time experimenting with dark-adaptation.
Yes, the effect described is well-known. I've also found something
that others might want to look for, I've found it documented nowhere.
If I'm, say, lying in bed looking at the ceiling, and my eyes are
dark-adapted, and there is a low level of illumination on the ceiling
-- which is painted white, and if I close my eyes, and then "flash"
the image of the ceiling by opening and closing my eyes once, very
rapidly, I find that I see what appears to be a dark object around
the center of my vision.
This is almost certainly the central scotoma, being directly
observed. It's known to be there. If you arrange an image so that a
small object is entirely enclosed within the central scotoma, viewing
with one eye, the object will disappear. Obviously, the seeing
mechanism fills in the scotoma, so that we don't notice it,
apparently it fills it in with whatever surrounds it. Otherwise the
scotoma, if we were to see what our eyes were actually detecting,
would be quite distracting!
However, under the conditions I describe, I think, the filling-in can
actually be observed. When the eye first opens, there is no
information on which to fill in the scotoma, so it appears, but just
for a flash. Once I knew what to look for, I could, if I quickly
opened my eyes, paying close attention, see the scotoma appear and
fill in, within a fraction of a second. It looked like it was
"collapsing." With the flashing, the scotoma is visible for a very
brief time as an after-image, it seems, but I'm not sure.
