BINOCULAR INSTRUMENT, an optical apparatus or instrument through which objects are viewed with both eyes. Such instruments can be conveniently dealt with as belonging to two classes : (I) Instruments f ot viewing solid objects or objects in space. (2) In struments for the presentation of plane views to both eyes for the purpose of ob taining a stereoscopic view of the subject, i.e., an impression of depth.
The natural vision is such that different central projections of the objects are communicated to both eyes, the differ ence of the two perspective representa tions arises from the fact that the pro jection centres are laterally displaced to an interval about equal to the dis tance between the eyes (the interpupillary distance). Binocu lar instruments should aid the natural vision with both eyes. If objects be so far distant that the two perspectives formed by the naked eye are no more distinguishable from each other, recourse may be had to binocular field glasses or telescopes, and if the objects be so small that in order to observe details on them the eyes must be brought so close to the objects that they cannot accommodate to the images recourse may be had to binocular microscopes and magnifiers.
The construction of binocular instruments dates back over several centuries and has now been brought to great perfection. The subject of their theory and history has been exhaustively treated by M. Von Rohr Die binocularen Instrumente (1907 ) the first publication to present a complete account of these instru ments.