We shall find the Cube of the Rainbow — Rainbow Trout

Lovers will boast their dreams, like the rainbow-trout they are angling. Cube and Arc are hard to explain if we don't expand the word rainbow.

We shall find the Cube of the Rainbow.
Of that, there is no doubt.
But the Arc of a Lover's conjecture
Eludes the finding out.

(F.1517/J.1484)

[1] Rainbow:: short for rainbow-trout _{(OED 3a)}; something beautiful but hard to get. Cube:: size in volume, not just length.
[2] no doubt:: people "shall find" the cube of the rainbow-trout, but they cannot.
[3] Arc:: curve of the trout; anything presenting this form optically or superficially, e.g. the rainbow _{(OED 3)}; something wonderful like rainbow. Lover:: one who is in love (with angling) and loses one's objective view.
[4] Eludes, finding out:: to boast the size.