What is the ring at the end of the engine in the [Douglas DC-8][1]

[![image][2]][2]<BR>([YouTube][3])


  [1]: https://en.wikipedia.org/wiki/Douglas_DC-8
  [2]: https://i.sstatic.net/8PwE6.png
  [3]: https://www.youtube.com/watch?v=XybBtck7cAU