There is disclosed an optical stereoscopic system having a prism window which permits free viewing of left and right steroscopic images by an observer. The prism window comprises a pair of prisms positioned with their long sides medially and their short sides laterally, and in abutting and joined relationship. Preferably, the prisms are thin plate prisms, each having a plano face and an opposite prism face formed with a plurality of parallel V-grooves to form, therebetween, a plurality of parallel triangular prisms. The optical system includes left and right stereoscopic imaging surfaces for display of normal, i.e., unreversed, left and right stereoscopic images. The prism window is placed in front of the imaging surfaces and a three dimensional image can be seen by an observer witnessing through a prism window.