extremes an absence of errors attributable to parallax is a further distinguishing feature of this embodiment again referring to fig3 if upper surface 14 and lower surface 15 are parallel and the base surface on which reference mark 2 resides is perpendicular thereto image 5 remains coaxial with reference mark 2 with this image location no error attributable to parallax arises between image 5 and peripheral scale marks 3 when the two are viewed by observer 11 the particular embodiment described above was designed and fabricated for the measurement of composite viewing angles during the human factors engineering analysis fo aircraft cockpit instruments and panels fig4 depicts a typical application in which observer 16 is seated before a panel structure 17 containing multiple instruments 18 at diverse viewing angles the design requirement prescribes that observer 16 must be capable of viewing instruments 18 from any point within head movement envelope 19 without exceeding specified angle limits between the normal to the face of the instrument being viewed and line of sight 21 in such an application the embodiment serves both as an angle measuring tool to establish reasonable standards for composite angle and as a test instrument to verify conformance to those standards to.