A combination comprising: a) a surgical microscope comprising at least one ocular port (40,50) and an objective lens barrel having an inner peripheral wall defining a cavity; and b) a drape assembly for maintaining a sterile field around the surgical microscope; the assembly comprising: an elongated hollow enclosure of sheet-form material having a first end defining an access opening and a second end defining a hole; a cylindrical frame located on the enclosure second end, the cylindrical frame having an inner surface and an outer surface, the outer surface of the frame being engageable with the inner peripheral wall of the objective lens barrel for mounting the frame on the barrel within the cavity; a window support removably mountable within the cylindrical frame, the window support comprising a resilient circumferential band and a plurality of spaced integral tabs, the window support further having an outer diameter substantially the same as the inner diameter of the cylindrical frame and an outer surface removably engageable with the inner surface of the cylindrical frame; and a window (210, 415) matingly engaged with the window support, the frame, window support and window (210, 415) sealing the hole in the enclosure when the window support is removably mounted within the cylindrical frame; wherein the engageable outer surface of the cylindrical frame includes: i) at least one raised resilient surface engageable with the inner peripheral wall of the objective lens barrel; or ii) at least one tab engageable with at least one recess located within the inner peripheral wall of the objective lens barrel, characterized in that when the cylindrical frame is inserted into the objective lens barrel with the outer surface of the cylindrical frame engaged with the inner peripheral wall of the objective lens barrel, the window is located within the cavity of the objective lens barrel. a) a surgical microscope comprising at least one ocular port (40,50) and an objective lens barrel having an inner peripheral wall defining a cavity; and b) a drape assembly for maintaining a sterile field around the surgical microscope; the assembly comprising: an elongated hollow enclosure of sheet-form material having a first end defining an access opening and a second end defining a hole; a cylindrical frame located on the enclosure second end, the cylindrical frame having an inner surface and an outer surface, the outer surface of the frame being engageable with the inner peripheral wall of the objective lens barrel for mounting the frame on the barrel within the cavity; a window support removably mountable within the cylindrical frame, the window support comprising a resilient circumferential band and a plurality of spaced integral tabs, the window support further having an outer diameter substantially the same as the inner diameter of the cylindrical frame and an outer surface removably engageable with the inner surface of the cylindrical frame; and a window (210, 415) matingly engaged with the window support, the frame, window support and window (210, 415) sealing the hole in the enclosure when the window support is removably mounted within the cylindrical frame; an elongated hollow enclosure of sheet-form material having a first end defining an access opening and a second end defining a hole; a cylindrical frame located on the enclosure second end, the cylindrical frame having an inner surface and an outer surface, the outer surface of the frame being engageable with the inner peripheral wall of the objective lens barrel for mounting the frame on the barrel within the cavity; a window support removably mountable within the cylindrical frame, the window support comprising a resilient circumferential band and a plurality of spaced integral tabs, the window support further having an outer diameter substantially the same as the inner diameter of the cylindrical frame and an outer surface removably engageable with the inner surface of the cylindrical frame; and a window (210, 415) matingly engaged with the window support, the frame, window support and window (210, 415) sealing the hole in the enclosure when the window support is removably mounted within the cylindrical frame; i) at least one raised resilient surface engageable with the inner peripheral wall of the objective lens barrel; or ii) at least one tab engageable with at least one recess located within the inner peripheral wall of the objective lens barrel, characterized in that when the cylindrical frame is inserted into the objective lens barrel with the outer surface of the cylindrical frame engaged with the inner peripheral wall of the objective lens barrel, the window is located within the cavity of the objective lens barrel.