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Re: [OM] Human Yellow-Eye Explained (Maybe ??)

Subject: Re: [OM] Human Yellow-Eye Explained (Maybe ??)
From: HI100@xxxxxxx
Date: Sun, 1 Apr 2001 22:53:40 EDT
 jlind@xxxxxxxxxxx writes:

<< Anyone steeped in knowledge about the human eye want to comment on 
 this?  Is this the source of it? >>
John,
    I have just been rereading "The Eye and the Brain" by Gregory and it has 
a color photograph taken with an opthalmoscope showing the retina. The Foveal 
area certainly is more yellow but even it has blood vessels and still looks 
reddish. The Foveal area is also pretty small at about 1/3 degree of arc. 
Seems unlikely to be big enough to cause this, even if it were fully yellow.
      An interesting number for the eye is the Total number of photoreceptors 
at about 120million.  A large number of which are preprocessed by the retinal 
nervous tissue so that they only detect gross image features (edges etc) and 
movement.  The smallest receptors in the foveal area are about 1 micron in 
size and with a similar spacing (approx 2 wavelengths of red light). The 
foveal area does not have that many receptors even though they are tight 
packed.
      Mamals and vertebrates have "inside out" eyes with blood vessels and 
nervous tissue above the photo receptors but cephalapods have the eye "the 
right way round", so go take some pictures of cephalapods for no red eye 
effects!
       Human eyes are interesting in that the stereo vision aspects are 
highly developed, but in most animals including some animals that have both 
eyes looking forward (not that common) the stereo aspects are not well 
developed. This is true for cats for example which have much higher 
sensitivity eyes. Some birds and primates seem to have the most highly 
developed stereo/depth perception abilities.  The speculation in primates was 
initially they needed good depth perception for swinging from tree to tree 
and then used that for developing manual manipulation skills.

                  Regards,
                 Tim Hughes
                >>Hi100@xxxxxxx<<

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