The Differential Surface Refractometer, and instrument based on the principles of surface refractometry, and modifications of this instrument as required to extend its usefulness to other glass types, is capable of determining the magnitude of the surface compression, in psi, present on tempered glass by nondestructive means. Since the strength of glass is dependent upon the strength of its surface, this ability to measure the surface compression of tempered glass enables one to assess the strength of this glass without the need for destructive tests. This report has demonstrated the correlation of the DSR readings with the strength of tempered glass of four different compositions: Type I MIL-G-25667 glass, PPG Polished Plate; Chemcor Glass, Corning Glass Company Code 0311; Herculite II Glass, PPG No. 7265; Alumino Silicate, PPG No. 6695.