NOVELTY - A gas detector comprises first and second sensing devices; an emitter; at least a first reflective surface that reflects at least first light rays from the emitter; at least a second reflective surface that reflects at least second light rays from the emitter; and gas entry opening(s). The first and second reflective surfaces are a mirror image of one another relative to a common plane.    USE - For sensing the presence and concentration of various gases.    ADVANTAGE - The detector does not require either electrical heating or large amounts of space, electrochemical choppers, or complex processing circuitry. The chambers used in the inventive detector can be inexpensive to manufacture, and require little or no adjustment or field maintenance. The inventive detector can be fabricated as a smaller, lightweight, stand along portable device that has no adjustments. It can, alternately, be part of a large monitoring system.    DETAILED DESCRIPTION - A gas detector (10) comprises first and second sensing devices; an emitter (32); at least a first reflective surface that reflects at least first light rays from the emitter, where a portion of the first light rays is received through the first sensing device; at least a second reflective surface that reflects at least second light rays from the emitter, where a portion of the second light rays are received through the second sensing device; and gas entry opening(s). The first and second reflective surfaces are a mirror image of one another relative to a common plane (P).    An INDEPENDENT CLAIM is also for a method, comprising providing inflowing gas sensing and reference regions at the same time; enabling the gas to diffuse between the regions across a common plane; projecting a sensing beam across a sensing region; projecting a reference beam across a reference region; sensing selected wavelengths in the beams subsequent to transversing the regions; and establishing a concentration of a selected constituent of the gas.    DESCRIPTION OF DRAWING(S) - The figure is an over-all view of a gas detector.    Gas detector (10)    Housing (12)    Sensing and reference portions (30a, 30b)    Emitter (32)    Sensors (34a, 34b)    Optical filters (36a, 36b)    Reflectors (38a, 38b)    Common plane (P)