Just a little flash of light.  Tolkien does not use the obsolete meaning in The Hobbit, but I’m delighted to note it here: a flash of eyes.

• 04.018 Now there came a glimmer
• 05.009 as he knew by the glimmer of his sword,
• 05.109 as Bilbo could see from the faint glimmer on the walls.
• 08.002 in a sort of darkened green glimmer.
• 08.033 They glimmered
• 13.044 A white glimmer could be seen

“glimmer, v.” OED Online. Oxford University Press, March 2015. Web. 11 May 2015.