Extensometry - The measurement of the deformation of a specimen under tensile loading, usually in connection with the tensile stress-strain diagram of a material.

External Ballistics - See: Ballistics.

Extinction - In optics is synonymous with optical density. The extinction per unit path in a medium is termed the optical extinction coefficient.

Extinction - In diffraction by crystals: (1) the attenuation of a beam of radiation by diffraction, as distinct from absorption. Primary extinction occurs with perfect crystals and arises from interference between the direct and multi-reflected beams, So that only a "skin" of crystal is able to reflect. Secondary extinction occurs with imperfect crystals and arises from the shielding of underlying mosaic blocks by' identically oriented blocks nearer the crystal surface. (2) the total absence of Bragg reflections from certain sets of crystal planes as a consequence of the symmetry of the space group. Such absences are also known as space-group extinctions.

Eyepiece, Orthoscopic - A comptes eyepiece with a wide field of view. The distinction between field- and eye-lens is here not clear cut.

Eyepiece, Ramsden - An eyepiece suitable for use with a graticule, consisting of a combination of two converging lenses of equal focal length. When an achromatized doublet eye-lens is used. thc eyepiece is known as a Kellner eyepiece.