Folklore has it that at the end of every rainbow sits an imp1, hammering on a shoe, who can, with persistence and guile, be forced to reveal to his discoverer the whereabouts of a crock of gold.
This is in fact no more than the truth...
According to Kermit the Frog rainbows are 'just an illusion2', but there is a lot more to a rainbow than there is to, say, a ghost.
For one thing rainbows are equally visible to all, can be photographed and are rationally discussable in a way that many 'subjective' experiences are not.
Furthermore, it is not enough of an explanation to say that a rainbow is merely the effect of sunlight on water vapour; the same can be said of how we are able to see clouds, but no-one calls clouds illusory. (Cloud illusions may indeed be more memorable than the real things: still, there is a real cloud, and there is equally real stuff out there wherever a rainbow is seen.)
Rainbows have an observable shape - they are circular (normally seen as an arc of a circle or of two concentric circles). They also have a rationally-discoverable shape in three dimensions, and this is where the insight begins.
There is an End to Every Rainbow
The appearance of a rainbow occurs when the Sun (or another light source) is reflected by many tiny water droplets positioned at a critical angle relative to it and the viewer. The shape of the reflecting droplet-group cannot be flat like the page you are reading (or the screen it's on); it has depth. The depth is not apparent because you are looking 'along its barrel', and therefore in three dimensions the rainbow forms part of a cone.
A cone is an open-ended shape with a single point. Mathematically a cone is complemented by its reverse cone, opening out from the same point to a second open end (circular, infinitely distant). The second cone makes no difference to the present discussion and can be retained in your mental image, or discarded, as you wish.
Where is the cone's point, when we are looking at a rainbow? Well, it would appear to be inside the eyeball, where the projected rainbow image is focused for communication through the optic nerve into the brain. But this is where the insight really begins: what if we are looking at it with two eyes? Has the rainbow got two ends, or are we seeing two different rainbows?
Stiff-necked scientists may wish to leave the rest of us here and enter the 'two rainbows' lobby. Resist any temptation to join them, as such a decision is damningly short-sighted. 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-projection3.
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 rainbows4).
It is in our consciousness5 that every rainbow we see comes to a single and perfect end. It is also in consciousness that all the values in the universe are situated. For such boundless riches 'a crock of gold' is a poor enough metaphor.