Re: Feline night vision - due to retinal rod/cone ratio or shape of iris?

Anne,
  Cats do see better in the dark than people, but the answer doesn't really 
lie in either their iris shape (although it helps) or their ratio of rods 
to cones.  The real difference is in the retina.
  Behind your retina their is a layer of tissue known as the choroid which 
is chock full of pigment.  Any stray light that enters your eye and doesn't 
excite a photopigment is absorbed by the choroid.  That's why your pupil is 
black; most of the light is absorbed and very little is reflected back out. 
When you shine a lot of light into the eye, for instance a photo flash, you 
do get some reflection from the blood vessels and photopigment, causing a 
dim redeye effect.
  But kitties have an extra layer behind their retina known as the tapetum, 
which is basically a mirror. So, any light that passes through the layer of 
rods and cones without exciting them will reflect off the tapetum and have 
a chance to excite rods and cones on the way back.  This is why a cat's 
eyes (or a deer's, for that matter) will reflect back a bright yellow 
light when you shine something in their eyes.  
  Now, this extra light bouncing around in different directions back there 
decreases a cat's ability to see details.  Cats only have about 20/40-20/80 
vision.  But since the rods and cones (well, just the rods, if we're 
discussing night vision) have almost twice the chance to catch a photon, 
the cat is more sensitive in very dim light levels.  
  Cat's also have very large pupils in dim light, as you noticed.  This 
also increases the amount of light that can get back to the retina, 
increasing their sensitivity.  But since light entering the eye far away 
from the pupil is much more subject to aberrations, the cat again trades 
off ability to discriminate detail for sensitivity.  If you've ever had 
your eyes dilated by an optometrist or ophthalmologist, you know that 
things are blurrier when your pupils are bigger.
  As for how the ratio of rods and cones may affect cat night vision, I'm 
not really sure.  I don't think the ratio of rods to cones is as important 
as the size of the receptive fields that the rods are a part of.  An area 
of the retina in which a photon of light will excite the same ganglion 
cell is known as a receptive field.  A single cone will often connect to a 
single ganglion cell in the retina.  So the entire receptive field is 
the size of one small cone.  If two photons strike two adjacent cones, 
and therefore two different receptive fields, your brain will be able to 
tell that the two photons came from two different sources.  This is the 
basis for your ability to see details.  Rods, however, are wired for 
sensitivity.  Often several hundred adjacent rods will feed into the same 
ganglion cell. This is a very large receptive field.  Here, if two 
different photons of light fall in that field, your brain won't be able to 
tell exactly where they came from.  But you have twice the chance to detect 
if there was a light or not.  You're more sensitive.
  So there's the long answer.  I hope I've helped out.  Write back if you 
have any more questions. 

Sincerely,

Tom