A tensile tester especially useful for colored specimens having pairs of projector-optic arrangements for detecting the presence of bench marks on a specimen and each including an ultraviolet lamp projector and an optic arrangement incorporating a detector for receiving luminescent rays emitted by the bench marks on the specimen upon being irradiated ultraviolet light from the projector. Support structure and a drive arrangement are provided for causing the projector optic arrangements to pursue automatically the bench marks controlled by detection of luminescent rays by means of the detectors in the optic arrangements. Elongation detectors are provided for transmitting respective displacement signals of the arrangements in response to the operation of the support and drive arrangements. An arithmetic unit of elongation is provided differences for receiving the signals from the elongation detectors and indicating the difference in spacing between the bench marks.
