Researches have been carried out to obtain a suitable apparatus and technique for field stress measurement in rock or concrete constructions by application of photoelasticity. The principle of this method is that a gage is beforehand fixed to a body whose stress is required to be measured. When the stress state of the body varies, there appears some stress in the gage. By observing the stress pattern in the gage by a polariscope, the variation in stress of the body can be determined. In the first place a polariscope and photoelastic gages suitable for this measurement were investigated, and three types of gages, i.e. a hollow cylinder, a solid cylinder and a rectangular prism of borosilicate glass and a portable polariscope, 500 gr in weight, were obtained. This polariscope has a detachable compensator so that it can be used for two purposes, namely to measure the relative retardation at an optional point on a gage by means of the compensator, and to observe stress patterns without the compensator. The stress in gages was analysed by the theory of elasticity to obtain fundamental data for finding a proper technique to determine variation in stress in a body. Referring to the results of analysis and taking into account the results of calibration, some techniques to determine variation in stress of the body that may be recommendable were obtained. Many experiences in practical application of this method of stress measurement have proved that the method is suitable for measurement extending over a long period of time
