h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

You say:

"In normal vision, the diverse images from left and right eyes are combined (by the brain) into a single image. Normally the combined image thus assembled, or revealed, is an improvement on the single image available from one eye alone, since it gives us a three-dimensional picture (though curiously, perhaps uniquely, this 3D revelation is withheld in the case of rainbows)."

Why? Because we do, in fact, see two rainbows.

When we see an object with both eyes, we see one object, but from two different angles. This allows us to work out its distance (by triangulation). And, if we see more than one part of an object, we can work out the three-dimensional shape of the object, because different bits of it are at different distances. Yes?

Rainbows are different because both eyes are looking at the same angle at different rainbows. And so we are not aware of their three-dimensional shape.

That seems to make sense to me, but I'm sure there's a flaw in it somewhere!

I've contradicted this extract from your Entry:

"If we really see two rainbows, we equally really see two of everything. It is true that two images are formed in normal two-eyed vision, but 'seeing double' is a pathological state, not the normal case. Another level to 'seeing' must be included, beyond eyeball-projection."



I've just had a thought that it's very hard, perhaps impossible, to judge distance on a featureless plain. Which suggests that it's not as simple as using triangulation (which would hardly be very accurate anyway, as the eyes are quite close together; though, come to think of it, our perception of distance isn't very accurate except at fairly close quaters, nor does it need to be).

TRiG.

Hi Trig. My assertion that we do not see two rainbows is an assertion about perception.

We see two rainbows in the sense that we see two of everything that we look at with two eyes. But that is not the normal usage, that is very much stretching a point.

Normal perception isn't complete until the unconscious brain has done its bit in sorting the input. At that stage we *see* one rainbow (or one set of concentric rainbows), not two, however many inputs it has taken to produce this perception.