Abstract: A binocular viewing device including means for resiliently mounting a prism seat to the binocular body within an enclosed chamber consisting of at least three elongated fasteners spaced about the periphery of the prism seat, with means associated with each fastener for resiliently engaging a first surface of the prism seat to urge the prism seat into fixed contact with the body, and at least one adjusting member extending through the body into the chamber from the outside, the inner end of the adjusting member adapted upon rotation for axial movement relative to the body for engagement upon a second surface of the prism seat in a manner to displace the prism seat from contact with the body, thereby to change the relationship of the plane of the prism seat to the line of viewing from ocular lens to objective lens for changing alignment of the optical system.