Complex sentence: These plates, executed from 1850 to 1854, are never found as a set and were never expressly published as such, but they nonetheless constituted in Méryon's mind an harmonious series.
Split into simpler sentences: These plates, executed from 1850 to 1854, are never to be met with as a set; they were never expressly published as a set. But they none the less constituted in Méryon's mind an harmonious series.