Let's summarize the main points, in this topic. And I think they're not hard to see, but I want to go over them briefly again, just to make sure that I drive them home. And the first one is that color is the name that we give to perceptions that are generated by different distributions of light energy. There's a distribution measured by spectrophotometers over the range of light. Remember, light is defined by 4 to 700 nanometers over that range. Of course some can be short wavelengths, some can be long wavelengths, some can be middle wavelengths. We expect a photometer lays that out for us. And the color is the name we give to the effects that arise from that different distribution. Second point is that color vision, it's fascinating. I mean, we have [LAUGH] spent a lot of time talking about it here in these lessons, and people over the centuries have spent an inordinate amount of time thinking about and trying to understand color vision, even though biologically, it's not all that important. Lots of animals, the majority of mammals for example, don't have the same kind of color vision we have. Lots of animals have no color vision, and it's not all that important, but obviously for the reasons that we've talked about, it does provide some advantages. And we at least, and other primates like rhesus monkeys, have gone to the trouble to evolve vision that gives us trichromatic color percepts that we have, that we enjoy, that rhesus monkeys enjoy, and that most animals, most mammals, don't have exactly the same color vision. The challenge, and it's the third of these points that I want to summarize, the challenge Is to explain how it is that we perceive these colors and the phenomenology, the odd way in which we perceive it. And the demonstrations that I've showed you, I think make this point clear, that just as in luminous, there's an enormous discrepancy between the physical metrics, spectral photometric ways of measuring light and what we actually see. They're just not the same. There's a huge discrepancy. And anybody who wants to say, well yeah, I understand color vision has to be able to give some kind of rationale for how that's happening. And again, I would argue, not everybody would certainly, but I would argue that the main challenge in color vision, just as the main challenge in luminance, is a way of explaining the discrepancy. That is, how we cope with the inverse problem, the fact that we don't have the physical information we need to know the world in terms of the way instruments measure it. We need some other strategy to do it, and the strategy that we use, I've argued again here in the context of color vision, is an empirical one.
