“Nessie,” whose existence in a Scottish inlet has been ardently debated for centuries, lurked as a shadowy presence within Gerard Byrne’s magisterial US solo debut, which consisted of a multipart installation clinically titled Case Study: Loch Ness (Some possibilities and problems), 2001–2008. Is Nessie a terrifying monster, a wind-generated mirage, or a folkloric myth? Is s/he a figment of the collective imagination, or an ingenious marketing stunt devised to boost newspaper sales and local tourism? And might there be a correlation between the increase in sightings, the rise of popular photography