To make a little bit more sense out of this,
let's look again at a picture I showed you before.
Which is looking at the retina through an ophthalmoscope but through
a microscopic ophthalmoscope that can actually reveal the density of receptors.
And you remember what we said before was that [COUGH] in the fovea,
in the central region of vision.
Where we direct our gaze when we want to see something in color,
with great sensitivity.
We point our eyes in that direction and see color well because
the fovea Is entirely made up of cones.
As you move away from the fovea,
the rods increase in number as we went through before.
And as you move into peripheral vision, the cones are sparse.
You could again easily show this to yourself
by taking a colored picture of some kind that has a colored checkerboard or
the equivalent in any picture that you might look at.
And ask yourself when you focus on a particular spot in the scene.
What's the color of the objects or surfaces that you see a little bit away,
a few degrees or many degrees away from the point that you're focused on?
The answer is, you have a lot of trouble seeing what the colors are, reporting what
the colors are once you move a little bit away from central vision.
Again making the point that you need cones to see color vision.
And you need these three different pigment types in the cones to allow you to make
the comparisons that are needed to give us our subjective sensitivity to color.
Now we want to talk about how we or
any individual reports color light or can report the colors of light that we see.
This a psychophysical measurement and you remember before,
I said that there are a variety of ways of making psychophysical measurements.
But one way that's very useful in thinking about color vision and
talking about it is a comparative wave.
So this diagram here makes a number of points and it's a bipartite screen.
On one half of the screen are directed
in an adjustable way long, medium, and short wavelength lights.
So this is the test side of the screen,
so to speak, the side that you adjust to set the stimulus that you want.
And this is the reporting side of the screen.
So you present, or the experimenter presents,
a color on this side of the screen.
And asks the subject to match a surface
by adjusting these three amounts of colored light.
And for almost all colors, it's possible for any surface that's shown
to an observer to match the two sides of the screen.
Based on the amount of long, medium and short wavelength light.
So this makes a number of points, the first of which is that yes,
what we said or what I said a minute ago is really so.
That depending on the amount of long, medium and
short wavelength light you can create any color that you like.
If you maximize the long wavelength you're going to see something that's towards
the red end if you maximize the short wavelength you're going to see something
that's toward the blue or violent end of the spectrum.
But as I said before, this is not just a simple matter of short wavelength,
blue long wavelength red.
As you know from the spectral sensitivities I showed you a minute ago.
This depends on the energies that are coming from any scene from
any surface that's presented to a subject.
It depends on those energies in a very precise way.
So this is a good way of demonstrating, number one, that it really is the mixture
of different light energies that can match, or that