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Ultraviolet vision

I know this is a bit off-topic, but since I write a lot about optics I
wanted to add a bit of clarification to something Mickey wrote:

>The fourth cone in reptiles is frequently a UV
>sensitive cone, so the animals aren't necessarily making finer
>discriminations; they're making discriminations over a broader part of
>the spectrum (Goldsmith, 1990).  That's just an obvious example where
>it's clear that not just the number of receptors, but also their
>spectral range is important for the conclusion Tom has drawn.  The
>fineness of spectral discriminations varies with location along the
>spectrum within individual animals because of this nuance (because of
>our peculiar photoreceptor complement we're particularly good at
>discriminating differences at wavelengths around 580 nm FWIW).

I'm not sure how much further into the near ultraviolet reptilian eyes can
see even with ultraviolet receptors because the lens, aqueous humor, and
cornea absorb strongly in the near ultraviolet. At 360 nm, 34% is absorbed
in the cornea, 12% in the aqueous humor, 52% in the lens, and the remaining
2% in the vitreous humor, according to Sliney and Wohlbarst, Safety with
Lasers and other Optical Sources, Plenum, New York, 1980, p. 107. I believe
those figures are for the human eye, but unless the reptilian eye differs
fundamentally in composition, UV receptors wouldn't get much UV light to
sense. (By the way, has anyone measured the peak sensitivity of this
"ultraviolet" receptor?)

It's this  absorption of ultraviolet light that limits the human visible
spectrum to wavelengths longer than about 400 nanometers. There are good
evolutionary reasons for the outer parts of the eye to block ultraviolet
light because it can damage the interior of the eye, particularly the
retina. I assume the same is true for reptilian and avian eyes.

An additional "ultraviolet" receptor in the eye could benefit by allowing
better discrimination between "colors" at shorter wavelengths, rather than
by adding a large amount of visible spectrum. I'm not an expert on color
vision, but I believe that what we see as "color" response depends on the
ratio of responses of the three different color receptors. Toward the edges
of the spectrum, these differences are small, making it harder to
discriminate between different wavelengths in the red or violet. Exactly
what the animals would see depends on the response of their brains as well
as their eyes.

-- Jeff Hecht, jhecht@world.std.com