He's going for the brass ring.
The eyepiece slides into the tube cd, which screws into the brass ring ef, through two openings in which the oblong frame, containing the micrometer slides, passes.
Original quality, 19th century, double handbell with a turned mahogany handgrip with a brass ring to the end.
Love this look for its brass ring details (suggestively placed on the shoulders, of course) and hardwire bikini bottom accents.
For most bikini models, Sport Illustrated's Swimsuit issue is the brass ring.