The denotation of 芒 is the prickle part of a ripped wheat seed. Its connotation is large number or amount. For example, 光芒 means millions of rays irradiated from a bright object.
The author (no more than 150 years ago I assume) of the passage was puzzled by the endless sorrow accumulated from his circular reasoning and wondered if there is an end to it or not (A modern homo sapien will see this as much ado about nothing).
In the example provided by Flaminius, 晦 = darkness, 芒 = infinite. Thus 晦芒 = (spatially) infinite darkness. 晦芒 could be interchangeably written as 晦茫 and both carry the same meaning.
Thank you kkmp for your to-the-point comment on 晦芒. Still, I'd like to reiterate that 芒 as an independent word can also mean dark/dim. CHENDICT v1.0 (c) Chineselanguage.org 1995-2007 defines it as below.
máng  awn  Chinese silvergrass  ray  blade  vast; numerous  dim