Complex sentence: In 1841 Tischendorf wanted to see and examine the codex, but he was allowed to read only Heringa's papers on it because Heringa was preparing his collation.
Split into simpler sentences: In 1841 Tischendorf wanted to see and examine the codex, but he was not allowed. Gregory Tischendorf could not work on the manuscript because Heringa was preparing his collation.