Murray does not state this point as lucidly as one might wish, so the quotations will have to skip around a bit.
Please read this brief (15 pages) and lucidly written report published last December by the Congressional Research Service.
1590s, "bright, shining," from Latin lucidus "light, bright, clear," figuratively "perspicuous, lucid, clear," from lucere "to shine," from lux (genitive lucis) "light," from PIE root *leuk- "to shine, be bright" (see light (n.)). Sense of "easy to understand" first recorded 1786. Lucid interval "period of calm or temporary sanity" (1580s) is from Medieval Latin lucida intervalla (plural), which was common in medieval English legal documents (cf. non est compos mentis, sed gaudet lucidis intervallis). Related: Lucidly; lucidness (1640s).