Newton had argued that a lens must colour the image if a prism colours a beam. Chester Moore Hall quietly glued crown to flint in the 1730s and got a telescope that did not. He never published. Dollond patented the same idea in 1758 and spent years in court over it.
Two glasses, opposite dispersion, one focus for two colours. The meter on this bench is there so the leftover fringe is visible. Cemented means the pair is one piece — no air to align, one less surface to dirty. Compare the air-spaced version.