the Lens towards X, and without Refraction, would there have crossed and
diverged again, may by the Refraction of the second Prism be reduced
into Parallelism and diverge no more. For then those Rays will recompose
a beam of white Light XY. If the refracting Angle of either Prism be the
bigger, that Prism must be so much the nearer to the Lens. You will know
when the Prisms and the Lens are well set together, by observing if the