The Spectrum Sings

A bit of a fill-in Blip today. I'm not quite sure what "the spectrum sings" is meant to convey as a message.
HOW many colors here do we see set,
Like rings upon God's finger? Some say three,
Some four, some six, some seven. All agree
To left of red, to right of violet,
Waits darkness deep as night and black as jet.

And so we know what Noah saw we see,
Nor less nor more-of God's emblazonry
A shred-a sign of glory known not yet.
If red can glide to yellow, green to blue,
What joys may yet await our wider eyes
When we rewake upon a wider shore!
What deep pulsations, exquisite and new!
What keener, swifter raptures may surprise
Men born to see the rainbow and no more!
William Cosmo Monkhouse