This internal diameter 1.375"/35mm cap allows you to protect most brands of Plossl eyepieces including Orion and older Meades (but not Televue plossls). It also fits Televue Nagler 4.8mm and Edmund RKE eyepieces. For a complete list see the eyepiece compatibility chart on the home page. It fits on the objective end of the eyepiece that you place your eye to. It is made of soft yellow plastic so it is quick to put on and take off, and easy to see in the dark.