Alight here means: "to find or unexpectedly see something"

It is most commonly used to mean to get off from a bus train etc, but it also has other meanings for your context. to agree on something, which is taken from the logic of the second example sentence in dictionaries to settle or stay after descending: "The bird alighted on the tree". "Historians had agreed or accidentally found some information about the mid 19th century that required explanation" to answer more fully a little more context is required; as to whether it was agreed or discovered accidentally or a combination of both.

Shed light... highlighted... discussed at length