What is this ring-shaped device behind this DC-8's jet engine?
(airliners.net) Ejector on the Rolls-Royce Conway of a DC-8.
It's called an ejector.
Introduced in c. 1958 by Douglas for their DC-8, it is extended during takeoff and landing, and stowed during the flight because it increases the drag at high-speed flight.
In the extended position it acts as a noise suppressor, and it also reclaims thrust that has been lost due to the corrugated exhaust nozzle. Doors within the ejector also close to provide thrust reversal.
(Flight—PDF)

Two views of the device developed by Douglas to suppress the sound and reverse the thrust of the DC-8. The corrugated exhaust nozzle is combined with a cylinder which extends beyond the nozzle during take-off. This &quot;ejector&quot; is claimed to reduce the sound level by 3-4 decibels and increase the thrust sufficient to offset the loss caused by the nozzle. The indentation near the forward lip of the ejector is one of the two doors which close to form a thrust brake. Noise level with the ejector ring and nozzle combined is said to be reduced by 9-12 decibels.

(Images from aamalebourget.fr and flightglobal.com)
The image on the right shows a similar installation on the BAC One-Eleven with the positioning screw-jack mechanism.
(YouTube) Factory testing of the ejector on the DC-8.