As I was about to say, when I was stymied by the Mudcat Orc :

J. E. CIRLOT in "A Dictionary of Symbols", 1962, says that : " ... and there are a great many cultures where the Bridge symbolises the link between what can be perceived and what is beyond perception. Even when it lacks this mystic sense, the Bridge is always symbolic of a transition from one state to another - of change or the desire for change ... The Rainbow being a natural bridge symbol, denoting the union of heaven and earth [ and yin and yang ]and the link between God and Man ... "

And who was that amazing boy-child who sang that "Rainbow On the River" song around Wartime England???

Cheers! R-J