I don't actually know much about ferrets' eyes, but I'm going to make an
educated guess here...  as well as try and make a simplified description.
 
One of the reasons cats can see so well in the dark is that the back of
their eyes is a reflective surface - this is also why their eyes get that
spooky reflective green if you shine a bright light at them.  This is in
contrast to our eyes, which get that awful red color you see in flash
photos.  Since ferrets are like cats (their eyes get that spooky green color
in bright light), rather than like us (becoming red), I'm going to assume I
can compare ferrets with cats, at least to some extent.
 
Basically, the receptors in the eye that are best in the dark, the rods, are
so sensitive that they can detect a single photon of light.  But the
receptors are actually a little bit in front of the back of the eye, and
they don't cover every bit of eye surface - that single photon of light
could enter the eye but miss the receptors altogether.  The back surface of
our (human) eyes are filled with lots of blood vessels - this is what makes
them appear red with bright light.  If a photon of light misses the
receptors in our eyes, it will get absorbed by the back surface, and thus be
of no use.
 
In cats (and, I'd guess, ferrets), the back of the eye is reflective.  If a
photon of light misses the receptors, it will hit the back wall, and bounce
back.  This gives it a second chance to hit a receptor, and, in a sense,
doubles the chance that that photon of light will be of use to the cat.  The
problem is that the photon might bounce back at a slightly different angle,
making its source seem to be in a slightly different location.  This makes
the cat's vision a little less precise with the fine details, or a little
"blurry" in comparison to ours.
 
Basically, there's a trade-off between sensitivity (being able to see with
less light) and resolution (being able to make out fine details).  Human
evolution has favored resolution, since we don't have the same need to see
in the dark, whereas the evolution of cats and many other night predators
has favored sensitivity.  This kind of trade-off occurs throughout the
visual system, not just here; but the rest gets into lots of little
neuroanatomical details that I doubt anyone really wants to hear about!
 
Since I'm writing, I might as well give the most recent update.   We've been
fighting a long battle with Fez - tarry, small, mucousy stools, so-so
appetitite....  We'd tried amoxi (about 3 weeks), Baytril (2?), Carafate
(1.5)... but now we think we've finally found the problem: campylobacter, or
proliferative bowel disease (aka proliferative colitis).  He's been on
chloramphenicol for over a week now, and I'm happy to say that things are
finally improving...  And, so, since his health was finally getting back to
acceptable levels, we took in 2 more guys.  Not for good; we're just
fostering two biters for Obadiah's (a shelter in Brockport, run most
tirelessly by Pat Nothnagle).  I was wondering what the problem was when we
had made it through the first 24 hours or so with only light mouthing, no
real bites - I'd expected them to be absolute terrors.  Fearing I would
become complacent, one of them (Floyd) saw fit to teach me a lesson!  I now
have 2 huge marks on my chin - Dave said I look like I was attacked by a
vampire with poor aim!  Ah well, I knew it had to happen sooner or later.  I
suppose I'm lucky - Floyd (at least) only seems to attack faces, not hands
or feet...  Of course, my battle scar is pretty darn noticeable at work.  If
anyone with particular suggestions for a face-only biter, I'd love to hear
them (other than to keep at it, of course - that much I already know!).
 
-Rochelle
 
(and Dave, fiance; Gruagach, deerhound; Fezzik, Falstaff & Shadow, our ferrets;
and Floyd & Butter, fosterees...  plus Sicard the iguana)
[Posted in FML issue 1281]