What is the round thing on the tip of an MD-90 wing? I like to pretend I know about airplanes, but have always wondered what the round things on the tip of MD-90 wings are. Here is an image of what I am talking about:


To be clear, my question is what the round, hole-like item is in this photo. It is on the trailing edge of the wing facing backwards. My current best guess is that it is the hole that the smoke comes out of when Delta is using the planes for sky-writing and doing other cool loop-de-loop type tricks. I have yet to see the smoke come out...
This is a logo light. The MD90 as originally delivered to Delta didn't have these.



This is the wingtip light arrangement on an airplane without the logo light.