Abstract: Gun sight apparatus for day and night viewing includes three units, a base unit including an objective lens assembly and a windage and elevation assembly. An erector lens assembly is disposed within the windage and elevation assembly. An aiming reticle is located between the objective lens and the erector lens at the first focal plane. A second focal plane is located behind the base unit. For daytime viewing operations, a daytime unit, including an ocular lens assembly, is secured to the base unit. For nighttime viewing operations, a night vision unit, including an image intensifier tube, is secured to the base unit. The face of the image intensifier tube is located at the second focal plane when the night vision unit is secured to the base unit.